When buds swell towards the end of their life, it's the ovule in the bract that is undergoing a change through not being pollinated, it swells and hardens alongside the innards or that there bract. The plants have stopped producing any more psychoactive chemicals at that point too. (needs some science done, but I'm convinced it's all senescence from that point).
I wouldn't worry about 'the swell' it just adds weight for cash croppers.

Them plants look cool in the pics, I;d be happy with them and thinking about tapering feeds off in the next week or two.
