I found the cause of mentioned by Mark Trapp. This is clearly a bug.

I was playing around with the console, trying to find a work around for this. Then I realized your follower's maximum carry weight stays the same at 300 (getavinfo CarryWeight). It is actually his/her inventory weight that is increasing, rather than the maximum weight decreasing. You can check this by using getavinfo InventoryWeight console command.

So, this is the cause of the problem. The developers wanted to make sure your followers always have default items on them to make sure none of them walk around naked. You can see this by using showinventory console command. For example, Lydia has steal armour set and a bow on her even if you don't see them when you trade with her.

The problem is that there is a bug that adds the some default items to the followers every time they get re-employeed by you. In other words, when you re-employee Lydia, she will get another bow in her default, hidden-away inventory. You cannot see this when you trade with her, but showinventory will show you the additional bow.

The only way to solve this problem is to remove all items from your follower and re-enable him/her. In other words, use following console commands in the given order:

1. removeallitems
2. disable
3. *enable

I found the cause of mentioned by Mark Trapp. This is clearly a bug.

I was playing around with the console, trying to find a workaround for this. Then I realized your follower's maximum carry weight stays the same at 300 (getavinfo CarryWeight). It is actually his/her inventory weight that is increasing, rather than the maximum weight decreasing. You can check this by using the getavinfo InventoryWeight console command.

So, this is the cause of the problem:

The developers wanted to make sure your followers always have default items on them to make sure none of them walk around naked. You can see this by using showinventory console command. For example, Lydia has a steel armour set and a bow on her even if you don't see them when you trade with her.

The problem is that there is a bug which adds some default items to the followers every time they get re-employed by you. For example, when you re-employ Lydia, she will get another bow in her default, hidden-away inventory. You cannot see this when you trade with her, but showinventory will show you the additional bow.

The only way to solve this problem is to remove all items from your follower and re-enable him/her. You can do this by selecting your follower in the console and entering the following commands. (Make sure you empty their visible inventory first, so you don't lose any "real" items.)

removeallitems
disable
enable