I used to have similar issues with selecting the correct monitor resolution. Now I make a square image in photoshop and physically measure it on the monitor to be sure - I know pretty serious professional graphic designers who have been caught out that way.

P.S. I'm assuming since you modelled on the monitor that makes him look thinner that the model probably is too wide, unless you've been adjusting your image posts to compensate (?)

 

PoserPro2014(Sr4), Win7 x64, display units set to inches.

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