I don't really know why it has come to this (100 channels of television? Cars? Office work?), but the phenomenon is starkly real, and as long as soldiers are drawn from a U.S. population getting heavier by the day our military will inevitably become less and less fit over time.
Having women in the military, with the concomitant lowering of fitness standards to accommodate them, undoubtedly contributes, but I would be surprised if it were a big factor.
I still see scrawny, ill-developed, asthenic and emaciated-looking young males everywhere.
In fact, I will say that I see ten of those, for every ONE in the age group 18-25, that is seriously fat, though rather many look like they get little physical exercise.