slideshow option

When I view my pictures (which are quite large) using the slideshow, in some of the albums, the pictures are showing up as the lower resolution (smaller) size, but in other albums the pictures are showing up as the normal resolution (bigger) size. Why is this happening? I would like them to always show up as the lower resolution size because the normal resolution size is way to big to use as a slideshow.
Also, is there a way to remove the slideshow link from an album that only has subalbums with pictures and no actual pictures directly in it?
Thanks