I personally haven't found that to be an issue for Lacey - even with a grazing muzzle and daily work, during the summer she keeps it chunky: fit chunky, but chunky.
BUT, if it were an issue, the same stuff applies. Add rice bran/beet pulp/hay pellets to the RB and voila. :)
There are no wrong questions! Ask away. :)
ETA: in the summer I might try alfalfa hay pellets initially for a really easy keeper, then add rice bran or beet pulp if it's really necessary. Beet pulp and rice bran are both much "better" at getting weight on than just hay pellets, for obvious reasons. And rice bran is even "BETTER" at getting weight on than beet pulp, for most horses. So I would start with hay pellets, then beet pulp if a lb or so of hay pellets makes no difference, then, last resort, rice bran.