How to measure your shirt size<br>Keep your body relaxed while taking your measurements. Do not puff out your chest, suck in your belly, or flex your muscles. If you do any of these, the measurements will not be accurate and the shirt will not fit.
Sucking in your belly will cause your clothes not to fit.
It depends when you suck in your belly as to whether it will affect the fit of your clothes.  I didn't specify when belly sucking in takes place. 
( z1 cause :ARG0 ( z2 suck :ARG1 ( z3 belly :part-of ( z4 you ) ) ) :ARG1 ( z5 fit :polarity - :ARG1 ( z6 clothes :poss z4 ) ) )