Dinosaurs were huge and plenty of them lived in herds, oh and elephants too.
Eusocieties happen when helping your cousins and brothers is as good of an option as helping yourself, which means when there are many of them.
Sacrificing yourself, your own reproduction to save a stranger is stupid, sacrificing yourself to save 2 brothers is efficient, as efficient as if you reproduced yourself.
Sacrificing yourself to save 4 first degree cousins is efficinet, or 8 second degree cousins or 16...and so on..
Eventually when you have 100'000 sisters like an ant, it's more efficient to die and save 100'000 sisters instead of being selfish and maybe produce a few offsprings, selfish offsprings which wont help you out when they grow up.
There's a reason many animals adopt close relatives whos parents died, theres a reason gays and lesbians did not go extinct and they are found in many species, because even if they don't reproduce,they help their family survive better. There's even evidence of gay animals protecting sisters and cousins from being raped, instead of leaving and building their own family, they stick with the old one and help out.
Big animals are already eusocial, just to a lesser extent...to make them more eusocial make them have more brothers and sisters.
You can even see it in human children, I don't remember which experiment, old memory....they found out that male children will often display self sacrificing acts in oder to defend their brothers if they are many, like stepping in front of the danger and covering the other siblings behind their back, but will cower and hide if they are only two...unless its a female....male kids will almost always self sacrifice for a sister.
Iyou have to think like an animal, animals don't care about evolution, they don't have plans for the next millenia, but selfish animals have selfish genes which produce selfish offsprings...which means that selfsih parents will have to compete with their selfish offpsrings.
Cooperating animals help one another, selfish animals eat their own children to eliminate the competition...literally.
When animals start having hundreds of offsprings at once or live long enough to produce multiple generations, then selfish genes always fail against cooperative ones.
Cause having possibly 2 or 3 generations of offsprings helping you is better than having to kill as many of your offsprings as possible to not be outcompeted.
This is not a choice made by the animal trying to figure out the most efficient way, it just happens that cooperative animals are more successfull and starve selfish animals or at least out reproduce them. And selfish animals are often more successfull in harsh environments like the deep dark sea or the desert or except when they find a good niche which allows them to prosper regardless.
Good rule of thumb is, if you have more 2 siblings then sacrificing yourself for their good will always be more efficient than building your own family, so it will slowly...really slowly develop your genes, passed by your siblings into an eusocial society. To speed up the process, just have more sibblings.
The only problem is, big animals have long pregnancies, so evolving eusocieties is not impossible but it is incredily slow and tedious, our planet is really young and our universe is also young...it might take so much more time to see fully eusocial gigantic fauna.
By the time an elephant has had 1 new generation, a cat produces two generations, cats live less but evolve faster.
Heck...for most of the time earth existed there was only micro fauna or animals as small as a lizard...big animals have just appeared on the universal clock, dinosaurs died half a second ago on the universal clock and life on earth was born yesterday...