Hey buddy. They are massive because there are many people which requests "Printable" sizes. What I'm doing in my games is to cut the locations into scenes or as you said downsizing them.
I'm trying to provide the highest possible resolution for my supporters and this is the downside of HD JPEG formats.
I'm still researching if I can find a better compressed format without losing the quality. I tried PNG and they were over 1 GB :D