I'm almost positive you and I have talked Philly neighborhoods before.
One thing to consider...Philly hasn't changed it's boundaries since the 1800's. All of the cities that are creeping up on Philly have...repeatedly.
There's multiple things to consider when taking into account a cities size, and for that reason I use the city limits, and not the metro area. Philly is the third densest metro area and has been for some time. Philly is 143 SQ miles...Phoenix is 450 SQ miles. This represents 11K people per SQ mile vs 3K people per SQ mile. Phoenix amongst other cities are in this irrational urban sprawl...it's city limits are ...of course it's going to eventually be bigger. But there are parts of Phoenix that don't even resemble anything like a city.
Completely agree with Philly attracting young professionals who eventually leave. Maybe they should expand their boarders like all of these other cities...I know the surrounding counties would hear nothing of it tho...So Philly can't grow...it's anemic in a sense in that it's infrastructure is not akin to growth, yet it can't share the wealth of the surrounding counties unlike these other cities.
EDIT: According to this article, from the same site you are quoting, dated after your article, it lists Philly as number 6. But it also explains the same logic I am referring to. Look at the growth, where Philly doesn't change it's borders at all.
Philly slips from fifth to sixth, as Phoenix rises