Sounds like something is a little broken on the servers, Content-Length should be the size of the response minus the headers, there shouldn't be based on any meta data really, but I've certainly come across on the fly optimisation scripts that don't send or send wrong Content-Length.
Doesn't really matter too much if you know the files coming across the network are of a reasonable size. Heck you don't need a Content-Length header at all really. It's not required.
But in general smaller page weight is a good thing, and images are the leading cause of bloat still: (sorry if this is a bit self promotional, don't mean it to be).
From an SEO perspective, probably not a huge deal, from a web performance, so a small SEO factor, can be a huge deal if images are massive.
From not asking your users to pay significant cash just to view your page, it can be sobering what those 10-20mb of images might cost some folks