well yeah they are bigger and bulkier..
the 10" is about 1.5m tall when in its cradle, and ide have to say given that the ota in the 12 is already 1.5m tall would be nearly 6 ft, also given that the base on the 10 is 500mm so the 12 would have to be 600mm at least. also the 10 is 14kgs the 12 is 21kgs. so instantly the tube itself is 7kgs heavier
As you said its all about getting enough light onto your eyes. for a normal set of peep holes a 6" will give you good views, but from what your saying you need the most you can handel.
High quality eye pieces are the only way to go for you but they are not cheap! Since i don't have too much experience with these kinds of ep's ill leave that to the professionals to comment on.