I'm starting a regular garden again this year, and rhubarb will probably be part of it.
My only guess is, bigger pot = bigger plant. I guess there could be other factors though,
like feeding habits, too much water, not enough water, too much nutes, not enough nutes....
I always grew my vegetables in an actual garden, so the plants never had size restrictions for their roots.
Then followed the feeding instructions on the nutrients package.