In the November issue of Threads there is advice on fitting a tee shirt for a larger bust.  I have found that one needs a dart. I follow the standard advice for adjusting a pattern for a larger bust, usually directed at improving a pattern for woven fabrics. It works as well for knits.  For a tee, I always place the bust dart starting at the armscye, ending at the wearer's bust point. Without this dart the larger bust creates stretch lines in the front of the tee.