Also, it helps when asking a question like this to include the URL for the site in question. That way we can have a look and see if we have the same problem or to examine the sites source to see if we can work out what is going on.
Many websites, especially vBulletin forum sites, display the default font too large.
If you really need a very specific example..
http://www.vbulletin.com/forum/ . That site looks wrong too. Practically every site I visit that runs vBulletin exhibits the exact same font problem. The image in my example is from
www.abxzone.com.
A specific example is always useful. I have never knowingly looked at a vBulletin forum site. I have probably used them, but not paid attention to what software was being run. So, giving a URL lets me see the site, examine the source and see if I can come up with what is happening.
Looking at
http://www.abxzone.com/forums/, I see it uses a CSS file. That appears to specify the fonts as
"10pt verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif;". So, if "verdana" is installed, it should be used. As verdana is part of the MS Core fonts, it should be OK. There are a couple of spots where "tahoma" is added in front of this list, so if that is not installed, there would be a difference to the Windows machine. But, this shouldn't be the problem as the parts that are displayed wrong do not use this.
The parts that are wrong are look like "<strong>Open Chat</strong>" and seem to be using the default page font (i.e. verdana). That suggests that Thomas' statement that there are problems with the antialiasing is probably the answer. If so, there is probably nothing you can do.
The last thing I can think of is if you have a vBulletin site that is OK, you could compare its CSS file with a non-working site for the differences. This might lead to an answer as to what is wrong. But, I suspect there won't be a fix.