Should you buy clothes sizes that match exactly to your measurement?

For instance, I have a tshirt in small that fits me. The brand for the tshirt says that for small, chest is 34-36 inches. I measured 34 inches with a measuring tape. I'm trying to buy a tshirt from a different brand but they list their chest measurement as xs= 34 inches, s=38 inches. Does this mean the tshirt itself is going to be 34 inches if i buy in xsmall (in which case i would look quite stupid in it). Is this size chart telling methe measurements of the clothes themselves or are is it saying :if your chest is about 34 inches wear xsmall, if it's closer to 38 wear small" ?