I kinda feel that this problem is part of the whole "eco-system" of problems if you like.
So, I kind of agree with you on some levels, I really enjoy the large battles, but you do come to realise that defeating an army of 2000 has absolutely no reward. Ideally wars should consist of many small skirmish battles and village raids along the border, with a handful of large battles, in which the victor of said battles will practically win the war allowing them to go unhindered to siege a settlement or be granted land in a peace agreement. This is where we come across the first problem of the eco system...there is little to no diplomacy options.
Secondly, this may cause wars to be shorter or lack the constant battles that fill up the game. Here the second problem of the eco system crops up, there is very little to keep you occupied outside of battles. So as of now, the large battles are what keeps us occupied with the lack of content elsewhere. If it's fewer, but more impactful, battles that you want, first we must address what the alternative is. We need a full rework of interacting with fiefs, a governing system, political manoeuvring/intrigue, useful political alliances etc. Just something that will keep us occupied during peace time other than trading grain or winning your 600th tournament.