Why are PSD files so big?

[Frans Waterlander]

And your arms are too short to box with the dog.

 

[LesPalenik]

If the file is in Prophoto mode and in 16-bits, it takes about 4 times as much space as in sRGB and 8 bit.

[digitaldog]

If the file is in Prophoto mode and in 16-bits, it takes about 4 times as much space as in sRGB and 8 bit.

On this end, both identical 16-bit files are the same size in either sRGB or ProPhoto RGB.

[LesPalenik]

Andrew, thank you for the correction.
As you say, there is no difference in the file size if saved in the Prophoto or sRGB modes. 
So the only way to make the file smaller is to save it in 8bits, which would result in half the size.