Are the condoms you are using long enough for you? Para-Goomba may have a point but it’s possibly not pre-cum, I have a theory that if they are not long enough fluid gets in from the top and makes them slip.
I am not small in any dimension but have never been confident of them staying on, I just put up with it and kept checking but this causes stress that you do not need. I also started to worry about the chances of STD getting in there. I have tried many different types and the only thing I discovered was that some brands are really too small and even they moved about when wet but also meant I couldn’t feel much - orgasm in a plastic bag - I know how you feel. I recently decided to get some some Durex XL after seeing them in the supermarket and thinking they can’t be that big. To my surprise they fit perfectly are not XL at all, they are 1mm wider and a decent length. I never bought larger ones before because I assumed they would be huge - marketing something as XL is a bad idea in my mind because they are not.
I haven’t had chance to try them out yet as I don’t use them with my regular [ex girlfriend], but I do feel I may have stumbled upon an answer.