Fitted clothes are probably the best thing that happened to style. I really can't imagine going for a shoot wearing a size 14 when am just a size 8 (imagine the horrific sight) or wearing a size 8 when I'm a size 14 *laughs*, nobody wants to see the bleeding eyes of a photographer *laughs*.
It's essential to wear clothes that conform to your shape with just the right amount of room to let you breathe, while glossing over the unshapely parts if required without adding bulk to your frame. It’s better to do this so people don't get the wrong impression of you been sloppy or you lacking good clothes and good sense of style.
Like the popular saying "Dress the way you want to be addressed" what you wear affects how you present yourself and how you feel about yourself. You tend to act more ‘lazy and sloppy’, exhibiting a poor posture, when we wear baggy clothes. This translates into you having low self esteem and a dirty lifestyle.
This in itself is a great reason to ensure that you wear properly fitted clothes. Fitting clothes does not mean suffocating in a slim fit if you are plus sized. It simply means wearing what works for you and looks good on you that allows you to breathe without you getting lost in what you wear.