My mag release is pretty firm on my new G19.  I have to press it pretty hard to drop the mag compared to my Sig P226.  Is this normal?  Will it loosen up over time?

Yes will wear in.

 
Agreed.


Tangodown makes a Vickers extended mag release that drop in if it continues to bother you though.

Yea, the tangodown on my G19 is real smooth / soft to press

 
Normal?  No, not in my experience.  I'd say it's because you're lucky!  The soft magazine release springs in all of my Glocks have cause me to accidentally dump a loaded magazine out of a Glock I was carrying 3 times in 11 years.  Twice the seat belt pressing the pistol into my side was the culprit; and once my holster got bumped, and the magazine suddenly dropped onto the concrete pad I was walking across.  

I don't need a larger mag release button than the one that comes standard on any Gen3 pistol.  Shooting Glocks as much as I do, and knowing them as well as I do has convinced me that the stronger the spring, and the more modest the size of the release button, then, the better it is for me!  If Glock, GmbH has finally stiffened up their mag release springs all I can say is it's about time!  13, 15, and 17 rounds of ammo is no lightweight magazine load!  As I said, I think you're lucky!