Do you move much in your sleep? If so the movement may cause the padding to become loose and work less effectively, your movement can cause the diaper to stretch slightly overnight and become loose and the movement can also move the standing leak guards and leg guards out of place, so what is a well fitting diaper when you go to sleep isn’t so well fitting by morning. If you’re wearing diapers that are already too big this is only going to exacerbate the issue; a diapers ability to absorb and contain liquid is still bound by the laws of physics and gravity so if they are loose they are not going to be watertight and contain the urine.
I also find standing up is generally one of the worst positions to wet in if you want to avoid leaks. If you’re standing up gravity is drawing all of the urine straight down to the narrowest part of the diaper (the crotch) so if there is any looseness at all around the legs the urine is going to find its way out before it can get absorbed, especially if you are flooding the diaper. Flooding isn’t only about the volume of urine expelled but also how quickly it’s expelled, in the morning you may be peeing with a higher pressure than at other times so if you’re peeing faster than the liquid can be absorbed this may be causing the urine to leak out, especially if the diaper is loose with gaps around the holes for the urine to seep out of.
I would try sizing down if you think the diapers you have are too big. Then try wearing a onesie or a pair of underwear over your diaper overnight to help keep it in place whilst you sleep and see if that helps at all. If you’re wearing cloth-backed diapers with readjustable Velcro tabs try tightening up the diaper in the morning before wetting as well and make sure that the leg cuffs are still out. Finally try wetting in a different position to standing, I find lying down on my back or sitting in a reclined positions so that gravity is directing urine to the back of the diaper the best position for avoiding leaks (although I’m female, not sure if that is the same if you’re male/ have different anatomy).