Bastian comes with 7 medium armor pieces equipped. That may account for why he leveled that weight a bit but not the others. Unless he is still somehow leveling medium without 5 pieces equipped...
As to why companions require 5 at a time to level when they only require 3 to unlock the lines and players only require 3 to unlock and 1 to level, and as to why the devs failed to mention that in the patch notes but only buried it in the Help/Tutorial menu (which they also don’t signal to players in game), your guesses are as good as mine.
Ok so they can't even gain skill without 5 of a type?
Basically no reason to wear different weights at all then, unless you just happen to have some good gear for that slot.
Bastian comes with 7 medium armor pieces equipped. That may account for why he leveled that weight a bit but not the others. Unless he is still somehow leveling medium without 5 pieces equipped...
As to why companions require 5 at a time to level when they only require 3 to unlock the lines and players only require 3 to unlock and 1 to level, and as to why the devs failed to mention that in the patch notes but only buried it in the Help/Tutorial menu (which they also don’t signal to players in game), your guesses are as good as mine.
How to level a skill line, both for players and companions, is also displayed in the tooltip that pops up when you hover over the skill line progress bar. That only goes for PC/Mac of course, no idea how it is on consoles.