Quick growth and quick evolution.
And you know, comedy.
Because I have a love/hate relationship with truly terrible movies, I submit Dragon Fighter (2003) which has a fast growing sample (only one of the scientists knows what it is/was) that breaks out and escapes, complete with a truly unbelievable amount of stupidity.
There’s a DS9 episode where they try to raise a baby Jeb haddar.
The Quatermass monster grew by absorbing things whole, not by eating them and growing in size. Presumably if you weighed all the humans, pelicans, cacti and zoo animals it absorbed beforehand, they would add up to equal the mass of the final blob.
Jem’Hadar, and yes, here it is: The Abandoned (episode) | Memory Alpha | Fandom
Which is arguably what all blob monsters do.
Yes; rapidly-growing-alien-lifeforms have two strategies to acheive rapid growth; they can absorb the biomass whole and incorporate it piecemeal into their bodies, or digest it and change it into their own form of tissue. Tribbles (for example) are very efficient at digestion, rapidly consuming quadrotiticale wheat and converting it into tribble tissue.
Quite often the rapidly-growing-alien-lifeform will gain mass from no known source. Alien xenomorphs are particularly notable for this, since they don’t even digest their victims whole, but rather wrap them up and extract blood from them while still alive; not the best way to gain body mass, I suspect.