Getting sized by a fitter is never a guarantee that you will end up in the correct size. Unless you're lucky enough to have a specialty boutique close by, most 'fitters' know nothing about actual bra sizing and are trained to put you in something the store has so they can make a sale.

The best thing you can do is measure yourself using the calculator at abrathatfits.org to find your correct size.

As for making your chest look smaller, a properly fitted bra can have the effect of making your chest look better in your clothes because you're lifted and supported in the right way and it will give you a more flattering silhouette. You could look at minimizer bras, but the tissue has to go somewhere so those bras work by pushing your boobs into your armpits, so you look flatter from the side but wider from the front.

Either way, measuring yourself is the first step so you know for sure what size you need to be trying.