In Skyrim Lydia’s starting gear is gone

She won't use Elven stuff because it's weaker than her heavy type Steel armor, I guess because she's skilled in Heavy Armor and not in Light Armor. I've given her Dwarven stuff and she equipped it on the spot myself.

As for why she did equip the elven helm, well she doesn't have a helm at all in her default gear so any helm is better than what she has no matter the stats.

Here's an answer that explains what might be happening (but this question isn't a duplicate).

As to what's actually happening, it's likely that your Aela is duplicating her Hunting Bow over and over again, until her inventory is chock full of hunting bows, to the detriment of everything else. This hunting bow (like her Ancient Nord armor) is part of her "default" inventory, and is therefore hidden as a result.

Without access to console commands, there's no way to get the extra hunting bows (which are reducing her capacity) out of her inventory.

You can reset their carrying capacity with the following 3 commands (with your follower as the target):

removeallitems

disable

enable

Be sure to remove any equipment you've given her before doing this, as it will otherwise be lost.