Definately sounds like thrush. So hope you are both being treated topically (usual prescription is Daktarin cream for nipples after feeds, and either daktarin oral gel or nystatin drops for baby after most feeds, if it becomes severe i.e. shooting/stabbing/hot needles in breast then the next step would probably be fluconazole).
Large nipples can 'somteimes' have an effect but not usually at this age, if positioning is good.
If you feed your baby on a pillow rather than reclining back and holding him, then it's quite likely that he's 'sliding' on the pillow and pulling you nipple longer.
A baby with thrush has a tongue that hurts, so it's a bit like you swallowing with a sort thoat - you know you want to eat but can't, which 'could' be a reason why he's 'nipple' rather than 'breast' feeding.
I wouldn't be inclined to take much notice of the pre and post feed wieghing, most scales aren't that accurate to pick up an extra ounce or two, and at this age a bae only needs about 3 ounces of breastmilk at a feed - and of course it might have been a 'snack' or his lunch rather than his big meal!
Do you switch him to the other breast when he comes off the first side? it's always advisable to offer the second side in case they want it.
It's impossible to 'empty' a breast, you can only take 78% of milk that from a breast, there will always be some left (it might be difficult to get hold of but still there). If you express after a feed you would get less than a baby would who is feeding effectively, as a pump can only 'suck' whereas a baby sucks but also milks with his tongue. Pumping after a feed for 5 to 10 minutes will help to increase milk production.
HTH