photoshop scratch disk size problem!

first of all, I'm not good at english.

I usually use photoshop when modifing picture that was taken by dslr. So PSD size is 100mb~150mb. After modifing photo, It become bigger that before. About 450mb~. And my photoshop ram usage setting is 20gb.

By the way, when I use photoshop as usual, error window opened.

= "c drive cache disk full" (not exactly)

when before modifing photoshop, my local disk c free space is 90gb

then I opened four files

my local disk free space is...

about zero....

I set my cache memory disk

why photoshop use cache memory up to 90gb, and over ??

just 450mb picture.... Isn't it too much?

I want to know why so large, and how can I fix it.

thanks