diferent sizes after rendering

Hi!
The pictures i work whith are all the same size, but after endering the morph into an avi the little movies have diferent size, though my rendering options whre all the same. Why can that be?
The pictures i work whith are all the same size, but after endering the morph into an avi the little movies have diferent size, though my rendering options whre all the same. Why can that be?