Problem with followers maxed on carrying capacity but not carrying that much
Solution 1:
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.
Solution 2:
I'm pretty sure there is a game bug on all versions you can use to circumvent the problem. When followers are ordered to pick up items they ignore their weight limitation. Put items on the ground then ask your follower to pick them up. You can do this by talking to them and saying "there is something i'd like you to do" or pressing the use button from a distance slightly outside of the speaking range.
Solution 3:
If you ask a follower to pick up items by pointing to them on the ground they do not go through the normal weight-checking process. This is normally ok. The only problem I have seen occur is when you (for example) have Lydia pick up an iron armor (the same as her originally default armor), then try to go and trade items with her to retrieve it. You will not be able to find it in her inventory. The 1.5 patch takes care of some of this glitch, but not retroactively, so if Lydia has previously picked up extra iron armors, she may have 3 iron armors in her inventory and you will not have access to any of them. If Lydia is killed you will then see all of her inventory. The link above does explain part of this glitch, re-adding Lydia's default bow, but that is in addition to this part of the glitch. On the PC you can remove the extra armors, but on the console you have to use the wabbajack glitch as described in the video on that same link. So, as long as Lydia is only picking up items that are not her default items (or whichever follower you are using) then you may/should be able to remove those items later.