I've found boots can sometimes develop a vacuum, though that's usually the inner CV boot because the inner joint can slide in and out (think of the boot as a plunger).
I'd try to massage it a little to try and pop it out, but I'd assume when you drive it the grease inside will heat up and expand the boot to normal (just my theory).
__________________
2005 4R Sport 4WD "The last of the V8s!" - Custom TIG'd SS Dual Exhaust - King 2.5" +2 LT. - ARB Front & Rear - 37's - Dana 60 - Build Thread
2005 Tundra 2WD Regular Cab V8 - Chopped Frame - Short Bed Swap
1977 Celica Liftback - LFX Swap - Build Thread