Human skeletons weigh around 25 lbs. You can decide for yourself, based on that, if you really think you're adding multiple lbs to bone density, long-term, through the use of ostarine. Ostarine increases bone density by increasing mineral content in the bones, so just like with fluid retention, such changes will be largely temporal. Diet is far more likely to effect bone density, and must be maintained.
I didn't realize we were talking about 20 lbs over the course of 3-4 cycles either. I don't know what level you're at lifting-wise, but if we're talking about close to a year of lifting (I assume this is what 3-4 ostarine cycles would be), then gaining 20 lbs of mostly muscle and some fat is not out of the question.
And I did use ostarine once. I don't talk about it because on the one hand, I got nothing out of it, but on the other hand, my source may have been garbage. I do intend to run LGD the next time I cruise.