They seem meaningfully different to me. A proper shield is notably larger than a buckler. A large shield is notably larger than a small shield. A tower shield is even more monumental.
See for example this:
https://www.iconspng.com/images/medi...al-shields.jpg
A buckler is like just the central boss on the round shields shown on the top row there.
TFT has a bit of grain problem, especially if a full shield is rated 1, a tower shield is rated 3, and later rules want to add 1 or 2 points for talents. The result is some people here arguing that a 0.5 lb Main Gauche might stop 3 hits per attack, the same as a 35 lb Tower Shield (but without the -2 DX penalty).