I think there are two reasons why caps from some members can be perceived by others as being too large/oversized, without the poster necessarily understanding why people are complaining.
The first, is that some members might be using 'Classic View' when viewing the forum, which reduces the width on screen for posts (and causes scrolling.)
Using Scottishbloke's post above as an example (mainly for convenience - apologies Scottishbloke, no offence intended.)
Classic view -
And 'Not Classic View' -
The Cap just about fits the screen, relatively neatly.
The laptop I'm currently using (and did those caps from) has a display resolution of 1024x768; which brings me to my second, more likely reason.
Different members perceiving caps as too big (or too small!) could be due to the size (or resolution) of their monitor/laptop/tv etc that they use to view the forum, and could well be unaware of how large caps appear on smaller screens.
My old laptop has a resolution of 1280x800 (and it's more Widescreen than the other.) I had it running long enough to do 2 caps with it, of the same post -
Classic view -
Non Classic View -
As a further example, I took a photo of the post on my mobile phone, whilst browsing using my Playstation3, on a 32inch HDTV -
So I'm guessing that either or both (presumed) reasons are what are causing the issue.
Different members - different screens. And the 'Classic View' switch.
I do absolutely agree that caps over c.700 are too big, mind, but that's mainly due to my laptop display (plus I have Classic View on - can't get used to the other one.) And larger caps can take ages to load, which never helps..