9 minute video saying people want something newer and smaller for carrying. Disclaimer....I did not watch the whole thing. I can't stand watching people talking into their iPhone.
Correct me if I'm wrong....it's ok.
His summary included 2 reasons people are returning Glocks to the sellers:
1. As you said, Glocks are bulkier than many (most?) alternatives. They are harder to conceal than other guns in the same class and caliber.
2. Glocks have specific safety features to keep them from accidentally discharging, but many who carry decided they prefer a manual thumb safety and/or additional mechanical safeties to prevent accidents. The only mechanical safety that the operator engages is the trigger safety which has proven time after time to be less than perfect when the trigger is accidentally caught on something.
I would add a third reason from my own personal opinion: ergonomics. not all hands are created equal, but I found that the ergonomics on certain Glocks make them difficult to shoot accurately. I prefer SIG Sauer pistols. I haven't found one SIG yet that I couldn't shoot better than a Glock. One reason I think is because the angle of the grip is not the best.
The Glock 17 grip is angled more downward at ~22* from square, whereas most others I shoot follow the 1911 gold standard -- ~18* from square. A Ruger Mark Series target and competition bullseye pistols are angled at ~55*.
In short, if you're used to the more natural angle of a 1911, you'd likely be shooting high with a Glock if you don't adjust to it.
I have foun the G17 and G19 seem to be easier to shoot than the smaller Glocks. I've tried the G21, but the grip size didn't suit me as well (.45ACP grip has to be larger than the 9mm grip).
Anyway, I'm not a Glock hater. I do believe the advantages of weight, cost and ease of operation make it suitable for most people -- just not "perfect."