Large grid mining drills' drill area is too small
The large grid mining drills are tedious to work with compared to the small grid mining drills because of their drill area compared to the size of the drills themselves.
This means a big limit to available ship designs since you'll always have to fill up one side of your ship with drills. If pistons worked better that'd be another solution, but I know how hard those are to fix.
As for balance, large ship drills require more resources and are larger, so it'd make sense for them to have a larger drill area. The extra storage they have doesn't matter much because of the increased size.