They both have an embarrassment of riches in terms of the squad so they're less likely to get fatigue injuries, players having to start every game in a tough schedule because other options are injured (causing further injuries) and needing to rush players back at times.
In terms of pure numbers, City's squad is smaller than ours. I think the advantage they have is 16-17 players who are at that same 1st team level, where dropping someone in has very little difference in quality. I think we are possibly at 13-15 players at the same level and some of that is how you rate Tsimikas, Gomez, Firmino, Keita compared to the 1st XI.
The other factor is that City seem to have a lot of their strength in depth in attacking midfield. Plus the way they play makes 5 of the positions relatively interchangeable with a big part of their squad. We aren't quite like that. We have depth but it's not always like for like. Or it's that we ask specific things from certain positions (e.g. No.6 position) that makes it hard to have an elite back-up player who can slot right in.
I suppose the point I'm trying to make is that City's squad is no bigger than ours but they way they can utilise their squad makes it seem like it's got more strength in depth overall than it potentially actually has on paper.