Could you have a look at this quick screencast video when you can and let me know what you think I can do to remedy the issue?
What we really need to do is take a look at your PM display to try and determine why these are displaying smaller than in the forum posts as this is not by design.
Could you create an account for us that includes access to PMs? And then - preferably - send a Om to that account so we can view it?
If so then please create an account so that we can log in and see the PM behaviour. Please send a PM on this site to both myself (Yellow Swordfish) and Mr Papa. And send a Pm to the account you create for us.
It helps if you can include links to not just your forum but also to this topic so we can get to it quickly to re-read what has been said. A brief reminder of the problem is also very helpful.
It seems we are limiting the size in PM display to 50px whereas on the forum posts there is no such limitation. I am not sure why except it is probably a historic legacy of the original default theme and when the templates for Reboot were created the addition of the size was not noticed.
I see you have a child theme so you could try removing the limitation. The templates needed are spPMMessagesView.php and spPMThreadsView.php and both contain a call to the function sp_UserAvatar().
This has a first argument that looks like this:
remove the size parameter so it just looks like:
and see if that fixes the problem. You will need to clear your browser cache before seeing any difference.