NOTE: I had to rewrite this after I found out what caused the problem.
At some point, Bethesda changed the food effects, so there's now a sated bonus that increases your carry capacity by a whopping 25% while you're not hungry. This also applies to mods you may have installed on your backoacks for extra carry capacity: while not being sated, these mods increase your carry capacity by 10kg (exactly as the mod description states), but when you're sated, you get another 2kg for free (corresponding to 25% of the initial 10kg, after rounding).
This should not be happening, since being sated may have an influence on your physical condition, but not on the size of any bags mounted to your backpack.