Compression vs. undersizing

[JamesSwim]

Some suits are designed for compression fit. Some people wear undersized suits for compression. Is it one and the same?

If you undersize, maybe leg openings get too small, but perhaps suits made for compression are made with stretchier fabrics and cut differently? (usually they are made for competition)

I do realize that you can't undersize with nylons suits, for example, since they don't stretch