Because it's designed for the web, and there's no reason to send 16-bit data, SFW converts for you automatically. The files are half the size after conversion.
JPEG doesn't support 16-bit and again, SFW automatically converts to 8-bits per color when selected as an option.
Thanks for answering Andrew. I appriciate that. Yes, in this case my 8-bit PNG is 532kb and the 16-bit is 2,3mb. They share pixel size, so that's compression. It's weird that PS doesn't have more flexible PNG "save functions" cause what ended up generating the 16-bit file was my OS, if you know what I mean.
I know that JPEG doesn't support 16-bit but I'm only referring to PNG here. I also checked the PNG-8 option in the Save for Web & Devices function, but that removed the RGB color space and made it Indexed Color like a gif. I've tried to find out what the max bit value for PNG is but it's not clear to me. Does the format support 64-bit? So if I had let's say Windows 7 I could have a 64-bit PNG by saving that RAW file like I did trough File-> Save As?
Photoshop can save a 16-bit PNG but SFW doesn't. Think of it as a automate method of saving files for upload to the web. There's no reason for 16-bit upload. The SFW functionality just doesn't allow you to save 16-bit PNG just as it strips profiles and metadata if so set. I don't believe PNG supports more than 16-bits per color.
If you want 16-bit PNG, just avoid SFW.