This is an important feature because it creates a quick visual tool to let you know what Inbox it's located in without actually opening the message or right clicking on your To: address. When using universal inbox like this, it's important, however the subject line isn't truncated because of this, the subject line gets truncated based of the width of your Sort Column or 'Messages' Column. In fact, even the inbox account name gets truncated when the column width is less than stellar. Simply enlarge the column width to an appropriate size that makes you happy. Otherwise, the only option you could do is turn off the contact photos which will give you an extra 100 pixels or so of space. You can find that setting in Preferences > Viewing > Show contact Photos in the message list
To answer the question specifically though, it's truncated because of the column width, alter the size and it will no longer be truncated.