Help with Sprint fit in heel???

[ruxerr]

I have been wearing the KSO Trek's for about a month now and I love them.  I looked at the sizing chart on Vibram and ordered a pair of Sprints (Grey and Navy) in size 42.  They feel good in the toes and everywhere else but the heel.  They feel fine, but looking at the heel they look too big?  They are snug, and I have read about the "heel bubble" but how much of a bubble is there supposed to be?  I am afraid if I go smaller than the toes will be too short, Also concerned with going for a run and then not being able to return them.  Help please?
thanks,

[Rana]

Mine are snug at the top of the heel (where the tab is) with about a finger's width (on the short foot) and a half-finger's width (on the longer) of space at the bottom.  Given that I wear the same size (W39) in both KSO and Trek, and those fit perfectly, I'm guessing that's an acceptable "bubble."

The key thing is how they fit elsewhere - do they feel comfy and snug, or do they feel loose and flappy?  If they fit comfortably and close without the straps tightened, they're probably okay.  If you have to snug down the top strap to keep them on, then they may be too big.

Compare the Sprint heel (short foot's on the right) with the KSO, here:

 

[dopie]

If the heel bubble is there while standing, it may be big. My heel bubbles show up when my feet aren't supporting my weight (eg sitting, lying). I always based vff size around my toes and forefoot; the heel just kinda hangs out.