FYI - DPI just means a bigger render, displayed in a smaller space(usually for printing purposes).

Show us a screensot of the Photoshop Image Size dialog.

Check the Dimension in Photoshop(Edit > Image Size.)  If you check Resample Image, and need to make the dimensions smaller (to match what you had in mind with Poser) does the DPI change to 300?

If not then you did'nt render at the size you want yet.  Next time you render, change Posers render dimensions(Or just change your Preview window size) to 4.167(300 / 72) * (times/multiply) what it is at now, and render.