Pigs, I’ve heard, pump in a large amount of sperm. Pigs! 
I read somewhere they have the lengthiest orgasms as well.
I think wild boars have corkscrew penises.
“I think”. Way to keep your cover, Katherine the Great.
Wow - nice “Clerks” reference!
I think most of these answers are forgetting this part of the OP’s question:
Elephants & sperm whales produce a large quantity of ejaculate, but they are also massively big animals. So the proportion is likely to be rather small.
I’d think the answer would be some very small creature, on a proportion basis.
[If the OP had included plants, they would probably win – they are more efficient than animals at this. The common garden tomato would be a good contender – the edible parts are the reproductive seed of the plant, and over a season they grow much more produce than the weight of the plant itself.
The days not over yet.
Самый лучший смех сегодня. Спасибо.
I can’t understand your crazy Moon language.
According to an article in Nature (Nov.8.2007). the males of Drosophila mojavensis looses 2-3% of its body weight with each eajaculation - and they mate several times a day.
Great Tick reference, Derleth!
And then one can erect a statue of the species and try to attract a male for a donation, but you cannot guarantee who shows up.
See pics of a moose mounting a bison statue in, according to another source, Big Sky, Montana., but Colorado has staked a claim.
I now understand why elephants evolved with trunks.
My vote goes to the Ostracod