why are a 4k and HD export the same file size?

Bit-rate is a per-second thing, not a per-frame-size thing. So how could two exports of the same sequence be different at the same bits per second?

This is why you need to change the bit-rate per second for larger frame-sizes ... as otherwise, it cuts down the details of the export to achieve the smaller bit-rate numbers. At least double the bit-rate of an HD export for a 4K export ... if not more. Test to check what you're comfortable with.

Bit depth is different, of course. That is how many bits per color channel. 8-bit is 256 bits/channel, 10-bit is 1,056 bits per channel of color information. Unless you have a goodly share of 10-bit material on the sequence, there's no reason to export a 10 bit file. Except of course if you're using the most excellent Cineform which only comes in 10- and 12-bit flavors, but never uses more 'space' than it needs in the file.

Neil