I think the reason is, that on most or many, of the servers, their missions have all DCS modules as late activation units, so they get loaded into memory when you load in. This is to avoid the massive load hits, when someone joins in a module that hasnt been loaded in yet.
The downside of this is that system and video memory is filled up fast, especially on servers that support all or most modules and have a big variation of ground units. Can limit this memory consumption by lowering your texture quality to medium (1/4 memory consumption) or low (1/16). Not total, but on every AI and player unit that is being loaded into memory. Downside of this is that cockpits become unreadable, especially the older modules. F-18 and F-14 dont suffer so much, and medium works fine on those for example.
Lowering pre-load radius can help on this too, but at the cost of potential bigger load stutters. Not sure units/players are counting here, think its only terrain/map related?
I do wish we could set cockpit texture quality separate from the other texture quality option. Then Id just run that low, and cockpit high for online. If I would do screenshots, videos, or just enjoy the visuals on a offline freelight or singleplayer mission, then run high/high etc. Oh well, maybe one day.