How long have you had him Nik? He does look underweight (males are generally a bit slimmer than females but he's definately a bit too slim)... Try some raw potato in his diet and a bit of protein in the form of chopped prawn or mussel (tmc do the Gamma irradiated frozen foods that are ideal)... as well as being general veggies some larger Loricariids actually scavenge off of animal and fish corpses in their natural habitat so protein is quite important. The potato should help him put on more weight. Check his eyes - If they look as if they have 'sunken' into his head then he may be Ill (worms, bacterial infection?). B)
I've had this plec about a month - after taking him from a tank far too small for his needs. As for his width he looks normal but his belly is definitely sunken.
Eyes look fine.
Therefore will try him with potatoe to see if this "beefs" him up.