Can you use a 21700 in a 18650 light?
21700 is both wider and longer than 18650. I doubt any 21700 would fit in a 18650 light.
21x70mm vs. 18x65mm. Nope.
I don’t think that covers all cases. Let’s start with length.
An 18650 comes in the flavors protected and unprotected. A protected battery is longer.
IMHO an unprotected 21700 is not much longer than a protected 18650.
So if you have a flashlight with a spring on both sides, chances are that a 21700 will fit.
It is only fair to say that 21700 batteries are 3mm thicker than 18650 batteries.
For that you need a body with a 3mm wider bore, thats 1.5mm per “wall”.
If you have a flashlight with a “meaty” body, maybe it can be bored out as much as you need.
Mind you, I’ve done that myself, but that one had indeed a very meaty body.
And yet I almost failed because the slot for the O-ring is usually the narrowest part of the body.
Which light, specifically?
Typically, no.
You can use an 18650 cell in a 21700 light though. 
Hydraulic press…
No.