So you don't have LuCI installed (i.e. you made the image smaller), correct?
I'm not sure what you mean.
In fact I own 5 of these devices - and there's threads discussing them - I've linked one. I would call that "knowledge". The issues with that device started to arise with versions 18 and 19.
Historically, making custom images before the existence of the Firmware Selector was more tedious - but as that became prevalent, so did varying image sizes. The second factor are users who just simply did opkg update && opkg install foo - hence using more flash space for the same software. This all plays a factor in the image size.