tell us the pixel dimensions of your sources and your sequence. Maybe the limitations of the resolution of the original clips. There are some tricks to eke out slightly better quality when scaling up clips but we're talking minor improvements... If you send the clip in the timeline to aftereffects and apply bicubic scaling, you're supposed to get better quality... I seem to remember there are some 3rd party plugins which are supposed to give you better results when scaling up, but I never thought they made much difference.