Both the folder list and the list of emails enforce a minimum width.  For a monitor with resolution 1280, these widths jointly add up to more than half the screen.  This leaves the space remaining to read the actual message too small for many html emails.

The folder list is particularly wide, approximately 1/3 of a 1280 screen, considerably wider than the longest named folder.

To reproduce:

Open geary.
Try to resize the folder list to make it narrower. 
Find it refuses to become narrower than some fixed limit.