I think both need a slightly more than the size bonus. I mean it's good but usually the planet with small grav well will not be in a place where it can be exploited as such. And the Larger grav well as well. Clearly the thing about small grav well is so u can use PJIs better, and larger grav wellw ill take logner to cross so it will slow down enemies if they have to cross via the grav well to get to phase lane on the otehr side. Both excellent, but you don't get to pick where these planets are
Well how about you give an little extra bonus. The larger grav well should support more Logistics (Maybe +4), while the smaller should give u maybe 2 tactical slots.
Anyway just a thought
Because as it is the small/laerge grav well bonuses don't playa big role. This would give them a bigger role!