Well I think I just found the answer, it is definitely due to size.

These things are absolute barrels for the capacitance that we need.

http://www.parts-express.com/wizards/searchResults.cfm?searchFilter=&srchExt=CAT&perPage=27&sortBy=3&layout=GRID&page=6&srchPrice=&srchCat=163&srchMfg=&srchPromo=&srchAttr=

I found some good selections for the 470uf though in electrolytic.

http://www.parts-express.com/wizards/searchResults.cfm?FTR=470uf&search_type=main&WebPage_ID=3&searchFilter=470uf&x=0&y=0

The axial type will be what I need. Are there any durability advantages to be gained by going with the higher voltage rated ones?

thanks,

Brian