3" to much?

ok... i bought a set of bbk chrome 1 5/8" long tubes with a 2 1/2" collector... my dad has this theory that since the collectors only 2 1/2", the whole system will only flow 2 1/2".... will i still be loosing a lot of power??? ive already got a 2 1/2" mac off road h pipe i can go get for 75 bucks for long tubes if need be, but that pro chamber sounds so sweet...