A better solution would be to add the filter hooks before the actual call in the template and removing theme when done meaning in your theme before you call your sidebar using dynamic_sidebar or what ever use this:
add_filter( 'embed_defaults', 'small_embed_defaults' );
dynamic_sidebar('my_sidebar');
remove_filter( 'embed_defaults', 'small_embed_defaults' );
and the same with your main loop:
add_filter( 'embed_defaults', 'big_embed_defaults' );
while(have_posts()){
//YOUR LOOP...
}
remove_filter( 'embed_defaults', 'big_embed_defaults' );