I can't find any specific reference to this, so I'm going to guess.
With the Inbox selected on the left, right click the bar just above the list of emails where the column headings say "from", "subject", etc.... choose Customize Current View. Inside the options, click the "Other Settings" button and make sure the fonts aren't set to something huge (8pt is normal). If that's ok, look in "Automatic Formatting" settings, cycling through each option to make sure there's no huge fonts selected there. And finally, if the two previous ones are ok, look in "Format Columns", cycling through the options making sure From, Subject, Received and Size are NOT set to "best fit".
If that doesn't work, I know of nothing short of "detect and repair" that MIGHT fix the problem.
And, if that doesn't work, you may have just found the one and only FORUM MEMBERS STUMPER question that we allow in each calendar year. You'll have to wait till next year to throw a new one at us. ;)