My understanding is that of all the types of cells in the human body, female eggs and male sperm are the biggest and smallest, respectively.
Granted they are unique in having only one strand of DNA, but is there a physical “explanation” for their unique sizes?
According to this article, they are neither largest nor smallest.
Taking the question as a more general one about their sizes, I would imagine sperm are small because they have to travel. Every unnecessary picogram is dead weight slowing them down. And, of course, they are produced en masse, so extra size requires extra resources to produce them, which might add up to something significant in the long run.
And eggs are big because being big makes it easier to them to be found by the spermazoa.
Evolutionarily speaking AFAIK it’s because of specialization into 2 different sexual strategies. Sperm are cheap and highly motile. Eggs are more expensive, don’t move much, but they have a nice dowry to get the offspring started. If we can imagine an ancestral population of similarly sized gametes then natural selection would push them into one or the other. It wouldn’t pay to be in the middle because you would be out competed by specialists on either side.
This foundational difference between male/female gametes has macro effects on male/female sexual strategy as well. Females with their small amount of expensive eggs become the limiting resource over which males compete. Not that there aren’t interesting exceptions, but that’s usually the case.
I don’t think the gamete differences have anything to do with reproductive strategies. Womb space is the limiting resource, and would remain so even if ovaries produced a constant stream of millions of eggs.
Womb space would be a mammal-specific limitation, and so might be a specializing extension of the scarce-egg principle, which I believe (WAG) applies to a wider range of animals.
Then again, when you get into the insect kingdom, for instance, eggs don’t seem scarce - one queen being able to produce millions of eggs doesn’t seem that far off for ants, bees, or wasps.
(And in googling the phrase ‘insect queen’, I was surprised when the first hit led to Lana Lang’s alter-ego in the Comic Books. 
)
On the subject of interesting sperm and egg facts among insects, fruit fly reproduction is relevant to this topic. A few highlights:
Fruit flies are 2.5 mm long. Fruit fly sperm are 1.76 mm long. The male ejaculates only a few hundred of them, which are stored by the female. The female lays around 400 eggs, so apparently most of those sperm are used (possibly making fruit flies the only creature on this planet that does not make God irate on a regular basis).
The link below is pretty cool. If you slide the tab at the bottom the picture will show the relative size of smaller and smaller things. There is a representation of the sperm and egg for comparison.
From the linked site,
This is silly. For the spermatozoa, they’re counting the length of the flagellum for total size, while they’re ignoring the axon for this neuron. A legitimate comparison between the two would include the 4mm (!) axon length, making the Granule cell some 130 times longer on that axis. Besides, I see other estimates of 5-8 um for the soma of the Granule cell anyway.
I cannot say whether or not there are smaller human cells than sperm cells, but they cannot be much smaller. A sperm cell is hardly so much a cell as a machine. It contains none of the standard cellular apparatus, has no ER or Golgi, no ribosomes, hardly any cytoplasm and only a handful of mitochondria. It’s simply a bag of DNA attached to a tiny motor, and coated with some particular enzymes. Really, there’s nothing left to remove from it but the DNA, and the only human cells I know of without any DNA are red blood cells, which are 50% larger to hold a bunch of hemoglobin. Really, you’re not going to find much that’s smaller than a sperm cell.
They are absolutely fundamental. There are entire fields of evolutionary biology that are based on examining the consequences of differential investment in male and female gametes.
As has been said, this only applies to species with internal development of young, which are a tiny minority of all species.
::reaches WAY back to high school memories about sperm and eggs::
::NOT LIKE THAT, PERVS!!::
Don’t eggs start to divide before they even “hook up” to mom’s supply system? Cell division starts when the jelly coat gets broken, so the egg has to have enough “goods” to supply enough somatic cells until it implants.
And I’m 99% sure that the egg contains all the extra-nucleic material that her three sisters gave up when the egg was formed.
This would be a lot more technical if I could google sex stuff at work safely…
In the case of mammals, yes, it starts dividing before it implants. And even AFTER implantation, formation of the fetal membranes required to obtain nutrition from the mother takes some time.
In the case of egg-laying animals, the egg supplies all the energy until hatching.
The article I linked mentions both the length of the flagellum and the size of the head, which is given as 5-6 µm. It is hard to see how you could miss that, since it is the sentence between the one that gives the size of the flagellum and the on that gives the size of the granule cell. While Donovan may certainly be incorrect in his sizing of the granule cell (though he does cite his sources), there is no reason to think he is comparing the entire sperm length to the diameter of the granule cell.
Also, do all granule cells have 4 mm axons?
My comment was based on the idea that we were discussing humans in this thread. If we’re not, there are certainly much smaller cells than sperm.
Spam reported.