I think it's naive to overlook a lot of Chelsea's recruitment at youth level as far as bringing in more physically developed players, however it's also too simplistic to say that that's the only reason that they beat United or have been so successful competitively, although it is a factor. As has already been mentioned they have a lot of skilled technicians that aren't necessarily physically superior, but I do think that these players are sprinkled in alongside the bigger/quicker lads, you're never going to see a small Chelsea side at youth level. It often looks like boys against men at times when I've seen Chelsea line up against our boys over the years, which also speaks to our recruitment as we have had a fair few smaller players in our sides in recent times.
You'd think the likes of Mount, Baker, Hudson-Odoi, Gilmour, Redan and Castillo and others will go on to forge good careers for themselves and although youth development can be unpredictable, if more doesn't come from the players coming through there in the coming years then it'd be fair to say that the club is not doing their job as well as they could do.