late but for further explanation:
1.4 and onwards, the canvas of your textures must match the same exact sizes of the vanilla sprites, otherwise it wont work. as others have posted, for different sizes, you must replace the vanilla xnb files for custom sizes.
it's not sure why this came to be, especially since 1.3+'s experimental feature with its textures didnt have this restriction in place. maybe to fix sprite anim bugs when re-logic was updating to 1.4? it's quite frustrating, especially to those with older packs wanting to update to the newer ones.