Ok. It sounds like your Paediatrician has assessed bub's weight needs to increase. My DD1 had also not regained her birthweight by 3 weeks, was dehydrated and needed formula topups due to a simple case of physiological low supply on my behalf. That may not be the case with you though, so you might want to seek advice from a lactation consultant in conjunction with your Paed. Your Paed sounds like they are wanting to get Bubs weight up fast and is probably concerned that breastfeeding is using up calories if they're limiting the feed times, but 10minutes in total does sounds a little too short, did they mean 10min per side? Did he use any means to try and ascertain how much breastmilk bub is currently getting from you? I am totally understanding needing to give formula in this situation, but if Breastfeeding is important to you, get advice now on this as your Paed is there for bub, but a LC knows more about boobs, so both can work together KWIM.

Did you mean that bub has been keeping down a bottle of EBM no worries of the same amount of formula? And that now bub vomits the same amount given at the same time of formula? If so, I would suggest it may be an issue with bub adjusting/not tolerating the formula. If you have been giving EBM, does that mean expressing the top-ups might be an option for you and using the EBM instead of Formula, because that would be an ideal solution both for increasing weight and for keeping your supply up?.

Other than that, it may be bub is drinking too much as Mad B said, they can just drink and drink with a bottle and especially if their little tummies are not used to taking that much and haven't stretched, they will bring it back up. I suggest (if you still feel you want to go with the topping up method) you cut back 10ml at a time until the large possets stop. Or that bub is taking in too much wind (as they can gulp more with the bottle) and is bringing all the milk back up on top of a burp (have had that happen more than once here), if that sounds like it could be the case, make sure you're using the slowest flow teat possible, and maybe even angle bub up so bottle isn't rushing in, it can help to prevent bottle preference sometimes too if you still want to breastfeed.

Just throwing some suggestions your way, hoping not confusing you. Ring your Paed if the large possets continue as it could be indicative of something else, like reflux etc that might have to do with bubs weight too. Hope this helps!