Rails have a certain width to them and lie more or less at the same elevation as the barrel/feed lips on most guns. A dual-feed magazine is just as wide at the top as the rest of its length, which could cause the gun to be wider in this area. Notice that the examples of double-stack pistol mags are for large, duty sized pieces and not pocket blasters. What was said about single feed making feed geometry easier is true as well, since dual-feeds simply don't constrain the rounds as long during the feed cycle. Straight feed is even better, with no feed ramp required, but tends to make guns really tall.
TCB
TCB