Hexadeci
Paranoid After Midnight
I'm no expert in corn snakes, but I think I've seen/heard of an anecdote of a corn snake eating its own shed. I'll link it if I ever find it again. But bottom line, as expert people have said, is that if the snake is growing, it must be shedding by necessity. You are sure it's grown? Then it must have shed, and you just haven't seen the shed. Maybe it hasn't shed 5 times; if it's growing slowly, maybe it's even only shed once or twice (say it shed just before you got it, then 2 or 3 months later, then nothing for another 2 or 3 months). If you are quite sure you haven't seen any sheds in the process of cleaning, maybe the snake really did eat its shed? If it ate its shed, then it's entirely possible that the shed had bits of substrate stuck to it (what substrate do you use?). Then the substrate could be causing an impaction, which might explain the enlarged vent. It's only a possible explanation from a non-expert, but it seems to cover all the facts, especially given the enlarged vent appearing around the most likely time for shed.