Volume versus psi

New here, hello everyone. Just got into the sport and I’m now considering a tank. My local dive shop does go to 4500 psi, but they said I’d have to leave it overnight and well they’ll do their best, they can’t actually get it all the way to 4500 psi. They suggested he steel tank of 120 ft.³ where they can Fill it up to 3500 psi while I wait with no problems. My question is this, does the extra volume equal the extra psi when it comes to refills for your air rifle?
 
The reason that he can fill to 3500 while you wait is that that is the pressure that he keeps stored in the shops' large storage tank or tanks. That pressure is slightly higher than the maximum fill for most scuba bottles. He is trying to sell you a huge steal scuba tank that, out of water will weigh a ton. If you already have a smaller 4500 rated bottle, keep that and leave it overnight. If you have a gun that you want to fill to 250 bar, forget the large steal one. If you have something that only fills to 2000 psi, any scuba cylinder will be adequate. Your question relates to converting high volume to higher psi. The only way of doing that is to compress it more.
 
Are you looking for a number of fills comparison between the 120 cuft tank @ 3500 psi and a smaller tank @ 4500 psi?
I used AOA's tank fill calculator:
(Gun / 180 cc tank - fill to 3000 psi from 1800 psi )
120 tank @ 3500 psi = 33 fills
-- comparable to --
52 cuft tank @ 4500 psi = 33 fills

Yes, I left off a decimal point. Memory failed me.
Nope, I have never seen a 52 cuft tank either.
 
I should also state that includes gauge too. My buddy got the same tank from the dive shop for $450 then he bought the gauge and hose for $120. Shout out to R&L, I was hedging and had multiple phone conversations with Matt who was patient with my novice questions. Great guy. I put a quick release foster male on the tank so I could use a micro bore to save air. Probably overkill but he put up with me through the whole thing. If I do get a top tier compressor, I’ll definitely go through him.