Pakistan does not have a "long" head start as you are trying to say. It is just 25 years before Bangladesh. Pakistan is busy in wars & instability since 1947 which is not the case with Bangladesh. Still Pakistan's economy manage to reach 264-300billion$ in 2014. Tourism in North Pakistan alone...
Bangladesh & Pakistan's comparison makes no sense at all.
Also i don't think Bangladesh can surpass Pakistan as Pakistan has a big land. Pakistan is always busy with wars & instability since 1947. Give Pakistan a peaceful decade & i can bet it will expand it's economy 20 times of what it is...