What I learned about how to solve the Symbols Appear to be different in size in between the Data View vs Layout View ??
One thing I have noticed when I do the labels on my maps on either Data or Layout View they tend to go big or small when you look at them. Here is what I do first is to set the scale you want in the Layout View. You can add the scale number to the list by scroll down to "Customize This List.. So you can add the scale number you want to set to. Then switch back to Data View and set the scale you just set in Layout View. You will see how the size results are in between the two view. So, I fixed it in the Data View and the switch back to Layout View. You can also use the bookmark to set your scale this way whenever you move in either Data View or Layout View, you can return to the original scale you like. This is also very helpful too.
Yes, I know it is pain but it really helps me to know what you can see the label size in the Layout View. This is a good one if you are working in small project like I do for Campground sites.