Or does it make no difference unless you train poorly and unbalance yourself more? Seems that a strong/big guy would have to exert much more force and do more re-learning to correct posture than if he had trained on a corrected posture from mewing.
More muscle will typically decrease flexibility. However as long as you focus on stretching, posture and making sure the muscular system isn’t too over developed in one area (like super strong pecs and a super weak back) you should be fine.
Strength training gave me an advantage IMO because my body is used to tearing itself down and building up again. This includes bone mass since weight bearing exercises help build bones. It has only made me more robust and I believe has helped in my mewing journey