Is the field set as Text?
Yes, it's text. If I set it to numeric, the results of the sort is then correct.
This causes a problem elsewhere. I have a find panel with a pop up menu showing the user all available size options so they can inventory can be found based on a size. This quits working if I change the size field from text to number.
I do not have customer requirements that the sort be perfect so this is not a big deal, but it's likely to occur if someone notices that the sizes are out of order.
Text fields with a custom value list to specify the sort order should work.