You suppose chunky buds with a good bud-to-leaf-ratio are the consequence of natural selection. I don't think that's the case at all, since most plants that develop flowers like this are the result of selective breeding, either by farmers that traditionally cultivate cannabis for hash production (as it's the case in Afghanistan, Pakistan or India for example) and/or - and probably foremost - by 'modern breeders': Better bud-to-leaf-ratio means more surface area with a dense trichome coverage.
In fact, when you take a look at older bud-pictures, you rarely see the degree of chunkyness you see today. Why? Because most breeders specifically 'design' they genetics to produce flowers that look like that. It's what 'the market' dictates!
If chunky and foxtailing buds were such an evolutionary advantage, you'd see A LOT more of them in nature. But you don't. It's actually quite rare