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In my experience, Tahari sizing is TTS (true to size) whereas Theory sizing is usually small...but it can be unpredictable as well. I own a Theory blazer in size 8, but I tried on another Theory blazer last year that was a size 4! Go figure....
Theory tends to run small IMHO. Tahari feels small also, but not as much so to me, and as NL said, it's more consistent. I think Tahari is cut for a certain body type, though, and it's not mine.
My friend who is a pretty pear fits Tahari tops and jackets beautifully, with her smaller upper body and little waist. When we shopped for her, everything in that section looked divine on her, whereas I looked like Bruce Banner turning into the Hulk. *ripping sound*