What are your thoughts on case sizes?

Personally, I think thickness really tends to be more of the important dimension when it comes to large watches. Certainly diameter matters, but different case shapes and even bands can really change how the diameter feels on wrist. 
I don't really consider size at all when I watch shop, especially because if a watch does end up being a smidge too big, I wear long sleeves for 2/3 of the year anyways, and a proper long sleeve shirt can really help a bigger watch fit better, in my opinion. 
I think maybe in your case with the Hamilton, it is the NATO making it feel big. You can feed the NATO strap a few different ways through the lugs, so that only one layer of the fabric is under the watch, and then you also won’t have that loop protruding near the top part of the case, it’ll be on the bottom of your wrist. If you just YouTube search “different ways to wear NATO straps” you will get a few 5 minute videos that show you the ways that make NATOs so much better for some watches. I think it looks good though, maybe will just take time to get used to. 
Personally, I think far more people than they realize can pull off upwards of 40mm case sizes. 

If you find yourself preferring smaller watches, go with it, tastes change over time. 

I started with 44mm watches, and have come down to wearing mostly 39mm watches. 

I know that feeling... I'm totally fine wearing a big G-Shock on my 6 in wrists. But not for other watches. Maybe because of the feeling that G-Shocks were meant to be big. They beg to be big and tough...

Doesn't hurt that they are fairly light compared to a similar size mechanical; which is where I feel most of the difference is between a huge watch wearing well & not. 

Part of the appeal with G-Shocks are that they are these solid indestructible pieces that can take anything life throws at them and the size is part of that. The fact you want your other watches to feel more elegant I think makes sense. I generally Buy watches of 40mm diameter or more but with the exception of my G-Shock and Casio beater they’re all thin enough to go under my cuffs. 

I have a few G's as well and some are fairly big but they are comfortable so I don't care 👍

Go with what feels right for you…if you feel bothered with a bigger case size go smaller if they offer, I once thought to myself in the beginning, “ I‘ll never wear a watch under 40mm those cases are girly “ and look now how I eat my words…your taste and preference will change go with it…

I have watches over 39mm and can't stand them. When not looked at I don't want to be reminded they are on my wrist. For a long time men wore 32mm to 35mm watches before the Jewelry and flex era. Now watches are classed by their monetary value as jewelry to flex.

I agree. Oddly I’m not bothered by the size of the G-SHOCK watches I own.  I prefer a 38mm case.  But am waiting on a couple of 41mm divers.   
 

I wore the Navy Scuba around my friends shop and had to pass on it.  The journey continues.  

Some Casios G shock comes big. All depends what rocks your boat. 

I guess if you are looking at divers they are supposed to be big(ger) for legibility and WR. As long as dimensions suit your attire, wrist size, work or occasion to which you are planning to wear it I'd say go for it. I believe that is why watches have categories such as dive, field, dress, etc. Most would wear one watch across different categories and that is fine too. But I believe we all have different levels of limitation on that aspect. I mean I wouldn't wear a big G to a wedding but could probably pull off a Casioak. 😂