That's very odd. Both the DIVs which are those two boxes are set to widths of 46%, so there should be plenty of room for them. I suppose the problem is caused by the long legend text at the top of the left box.

One solution would be to turn off the two-column log-in in admin. That would put the log-in box for existing customers just above the registration fields for new customers. You can see that layout on my retail site (link below.)

admin/Configuaration/Layout Settings/Use split-login page set to false.

Rob