How it work Atlas made from sprite packer?

Hello.

While making 2d game, I confused Atlas made from sprite packer.

At first, I understand Unity5 Atlas was combined image resources either NPOT or POT to transfer POT image.

For example, Combine 120x40, 120x60 to 128x128.

But I'm not certain that correct.

Because I select Sprite Resource on ProjectTap, it be written ARGB32bit(original) RGAB8bit(atlas) on InspectorTap.

As I know, As in NPOT image was compressed 32bit(or not be compressed), and POT image was compressed select format(DXT5, RGB16bit...).

However, when I packing POT image, it be written ARGB32bit(original) RGAB8bit(atlas) on InspectorTap.

Although I select other compress format.

If Sprite packer combined image resources either NPOT or POT to transfer POT image, I considering change my resource POT to NPOT.

Because now I use only POT image, but it has much waist memory.

If I use NPOT, just need 120x60 image. but If make POT, I need 128x128 despite I just need 120x60.

Anybody give me some advice for clearly understanding?

And sorry for my english.