"Could the disproportionate thigh morph be down to the density of the base mesh?"
Klutz - I believe this could be the reason. When I'm working with V3 I tend to use 0.5 on the Vastus Lat. I had to only go with 0.3 on Aiko otherwise she just didn't look right, the morph was just too extreme. Another thing I noticed as well was when changing her hip size. If I reduced her hips after morphing her leg muscles, they went out of sync compared to Vicky. Mind you I suppose we have to expect a number of differences based on the differences in mesh.