Texture sizes too big and not changing no matter what I do.

I'm having a problem understanding what Unity is doing with my assets. At the moment I feel that my build sizes are too big. The Editor Log is reporting that it's over 200Mb. So I'm trying to optimise.

One problem is that the game has a large number of images and some of these images are being reported in the Editor as using more than 2Mb. However, the original images are 512 x 512 in size and take up less than 200Kb on the hard drive. I've tried playing around with import settings but nothing seems to be making a difference.

I've set the Max Size to 512 and tried setting the format to both 16-bit and then Compressed. But the Log's reporting no change to the amount of space taken up by the texture. Still more than 2Mb.