Originally posted by Bob 256 you might be inclined to go full-frame which cost more, are larger, and in less supply than their APS-C counterparts. For compactness, you can't beat an APS-C using an APS-C lens.
The price of lenses has more to do with the class of lens than whether it is FF or APSC. DA* 11-18 vs DFA 15-30 - DFA is only slightly more in price and it is heavier but over all close. HD DA* 16-50 PLM vs DFA 24-70, DA* is slightly more and DFA weighs only 62g more. I see this argument all the time but it doesn't really hold. Quality glass costs regardless of FF or APSC. That being said there is lots of good affordable glass for either format and lots of expensive glass. Picking the lens you want most and then building a system around it is a good way to go. I still think the K1 offers more flexibility as you get APSC and FF in one camera.
FF systems can get quite a bit heavier depending on what you choose, but this argument against the K1 often baffles me. My 10yr old son can walk around for hours with a K1 and 70-200 on it, yeah it's a heavy combo but hardly debilitating. He will complain after a while and try to get me to take it from him, but he will do that with a 200g frisbee because he's a kid. Even on long hikes I am generally carrying far more weight than a K1 so for a photo session the weight is even less of an issue.
The OP mentioned not liking heavy glass so compact primes are plentiful in the Pentax ecosystem.
APSC - DA Limited 15mm 21mm 35mm 40mm 70mm or the DA 35 and 50
FF - DA 35 and 50, FA 35 and 50, FA Limited 31, 43 and 77, DFA 50mm Macro DFA 100mm Macro
All of those are compact and light and most aren't that expensive and there are many more older used options too.