D15b vtec for honda 1.5 litre?

ok so heres my question may sound stupid but ive been readin alot about engine lately gettin into my first project car. but i guess im not understanding why is it certain engines can only get so much horsepower for instance some people talk about how the b series (if had the same done as the d series that it would have way more hp but it is only .1 litre bigger i understand that it stock has more horsepower but what actually stops a car from achieving anything higher then what people say itll get is it internals? ive heard block will blow is there any way to reinforce that? ive also heard reinforce cylider walls but that didnt seem like what people thought it was but i guess i dont understand why a car cant acheive something higher. ive heard the d15b can only get max of a few hunder maybe 250 ish at the wheel but why not any more?