Lexi's problem (apart from being a tight Jock!) is he can't physically get the small spool on the Portamig's large spool holder. There is an adaptor on the market that I've seen, but I've yet to source them economically.
weldequip
Weldequip - don't read this.
You can botch the Portamig. A reel of gaffer tape is an ideal spacer for a 5kg roll. I'm sure I pulled the tensioning mechanism off to run a 1kg roll too - seem to remember the roll fitting over the tensioner mounting shaft but can't remember how I tensioned it up. Half remember using a bit of Clarke tensioner, the gaffer tape, and holding it all on with the lock nuts.
Number of suppliers have adapters for fitting the little reels onto the 15kg reel holder. Search around - sure DIY Welding or Weld UK had one, though it might have been the evil migtigarc.
EDIT: Winding onto a big spool might make it tangle a little as it comes off, though knowing that you'll probably find a winding pattern to avoid that. Just seems a lot of hassle.