Because they're meant to be virtually invisible under tee shirts and other knits, t-shirt bras are often completely seamless, which not always the case in bras meant to minimize. But there are some styles that incorporate minimizing construction into a totally smooth silhouette.
Minimizer bras have cups that redistribute breast tissue more towards the sides of the breast. This redirection reduces projection and gives the appearance of reduced size. Modern fabrics like microfiber create an incredibly smooth and soft material that can still be rigid enough to provide support. The combined benefits of redistributed breast mass and ultra-smooth fabric creates a t-shirt minimizer bra that lays smoothly over clothing without bunching or gapping.