I think I've found a solution:

De-alias large_breasts from big_breasts
De-implicate hyper_breasts from huge_breasts

Give the large_breasts tag the current definition of big_breasts.
Define big_breasts as large_breasts, huge_breasts and hyper_breasts collectively.

Implicate large_breasts to big_breasts
Implicate huge_breasts to big_breasts
Implicate hyper_breasts to big_breasts

Start a tag project to clean up inaccurate tags.

For misc. loose ends:
De-alias large_boobs from big_breasts
Alias large_boobs to large_breasts

Individual sizes can be searched and excluded just like any other tag (solving the issues I presented). The implication chain becomes an implication... fork, I guess. Searches that relied on the chain can be adapted to the fork and much of the convenience should be retained. Tag number inflation via implications would be reduced (not an issue per se but it's still something).

Any questions?