Help me understand

OK I have a 180 gallon tank with huge sump. I just started dosing 2 part last week. I don't have a whole lot of sps, but I have noticed something about this tank that I find interesting. I don't have to do much to keep the water chemistry in balance. most of the corals I have are sps mushrooms and zoas. I do have a few sps in the tank. Than being said I also have jbj 24 gallon tank, that has zoas, palys, mushrooms and a ling si monti in it. The tank doesn't have a skimmer because there is no real bio load on it. I am amazed at the difference in the jbj it is literally double to triple the work to keep the chemistry in check. for example
both tanks the 180 salinity stays at 1.025 and about every 3 months it drops to 1.023 the jbj i 2 days jumped from 1.025 to 1.325 due to evaporation. I have to constantly monitor the jbj to make sure everything is stable. Here is my question why is it this way? Is it because of the tank size? Does everybody with small tanks have this happen or am I just the lucky one?

Suiso  it really is a % thing.  When I had my smaller 75 gallon tank I would notice changes much faster than my 220 gallon. 

suiso man wrote: Ok, so it is the amount of water in the tank then. That's what I thought it was. It makes me think that if your going to get a frag tank it should be a big one so that the corals aren't stressed with sudden changes.I like big tanks.I really need to make a fish room... do I dare ask my wife lol Thanks for clearing things up I appreciate it.

Or you need an ATO System.