those food will indeed go through them very fast.
Seeing mine will devour a quarter of a zuchinni every feeding time.
Very good pics to show the indentation and you're right, if it is caved he could eat some more.
But when a pleco is growing fast it will use up the supply of fat that they accumulate in that indentation.
On the whole the fish looks healthy to me.
plus there's a difference in nutrition between the different kinds of food.
Cucumber concists of mostly water.
That's why i feed the zuchinni to my fish.
keep tossing in some wafers some hours after lights out.
Allthough i found my SD's have an even better nose for the wafers and they start looking for the wafers immediately after i throw the wafers in.