Currently, big_top_small_bottom is aliased to big_dom_small_sub but the two are not actually the same. For example, you can have a smaller powerbottom. See this picture (https://e621.net/posts/4615574) for an example of which big_top_small_bottom applies, but big_dom_small_sub does not (and note how small_dom_big_sub is used instead). For this reason, big_top_small_bottom should be unaliased from big_dom_small_sub.
As precedent, we can use the same reasoning as given on the page small_top_big_bottom
Also brought up by this user 3 months ago https://e621.net/forum_posts/393091.
If approved, people are currently using small_bottom_large_top for this purpose anyways, so that could be aliased to big_top_small_bottom.
Updated