Like the title (and tags) ask, I'm missing something... I do quite well up until I get into the over 300/320 ton area ships, with payloads of generally of at least ~60 tons (the payload is far less heavy once trying to return from my destination, like maybe 15 tons or something).
Obviously, I must not fully understand how dV is calculated or works, because when I get to this point (I always avoid this, I try and make my crafts as minimal in all aspects as Kerbally possible) it comes to a head, and no matter what I do, I can't increase my dV any further...generally not past 3k for the lower stages. My upper stages are fine, just very heavy. My lower stages start becoming inefficient and massive amounts of fuel will only add like 100dV, an obvious indicator I'm Doing It Wrong™.
This specific monstrosity is a result of combining two difficult contracts into one mission, as well as trying to bring 7 kerbals all at once to Minmus for the first time. I have a very similar craft meant for 1 kerbal to return that works just fine.
I can add some pictures in a few hours when I get home if that helps, but I mostly want to understand why I'm doing it wrong, not just how to improve this specific rocket. In the past in similar situations, if I post a picture I tend to only get help with that specific rocket, not how to prevent issues in the future as well.
Thanks KSP