Thanks for the replies, all. I was hoping I wasn't going crazy in thinking there was no way he could need 20oz. in 10 hours. I used the kellymom website's guidelines for how much milk EBF babies usually take in a day, so I felt fine with what I was sending. My daycare feeds on demand, which is good - but having 5 feedings during 10 hours is definitely more often than we would feed at home. His first day, I sent him with 4 bottles of 3oz. each and a 4oz. bag of frozen milk. I didn't think they'd even need the frozen bag, but sure enough, they used it. That was the last time I gave them any of my freezer stash! Since then, I have been dividing my pumped milk into 5 bottles, and honestly I think he's fine with it. He isn't reverse cycling at all - in fact I've been pleasantly surprised that our nighttime feedings are exactly the same as they were before he entered daycare. So I don't think he comes home hungry everyday! And the daycare ladies are always telling me how little he cries and how well he does everyday. He always seems happy when we pick him up (I've never seen him cry at the end of the day).
I think you're right that they are probably more familiar with formula fed babies. My own mom, who never breastfed me, suggested that if DS always finishes a bottle, that means we should be giving him more. I have a feeling the daycare is making the same assumption. My assumption has always been that he just finds bottle feeding really easy and will guzzle down whatever we give him. That doesn't mean he needs more!
I think I will just politely tell the daycare that we have tried giving him 4oz. bottles but find that he doesn't eat that much. (That's only partially a lie - we have given him 4oz. bottles, and he will eat all of it and then inevitably spit a bunch right back up. So I figured that meant he didn't really need the whole 4oz.) I do give him a long nursing session right before and after daycare, so I will continue to do that.
Thanks for the help and reassurance!