I found this:

"The 915F has a full pear profile and comes in at 175cc, giving max forgiveness. The 915Fd is slightly smaller, coming in at 160cc and is more workable. The standard model is available in 13.5°, 15°, 16.5°, 18°, and 21° versions, while the Fd can be had in 13.5° and 15° versions."

I have not be able to determine if the 18˚ and 21˚ heads are smaller as are the 913 F's.

Yes, the F has the bigger head in the 915 series. With the 910's, and the 913's, the F was the smaller, more workable one. The Fd was the a tad bigger. You'd be impressed with both the F and the Fd. If possible, I recommend getting a hold of your Titleist rep to schedule a fitting so you can try some of the new heads/shafts, etc., out. Best of luck to you.

I have the 913Fd for the purposes of hitting off the tee, after just being fitted at Oceanside TPI I noticed that the 915 Fd was not a noticeable difference to my eye plus the spin is higher on the 915 F according to my fitter Steve.  I went with the 915Fd to keep spin lower off the tee and also it looks gorgeous!

Good Luck!

The 915F and 915Fd are both great with looks, feel, and performance! I went into my fitting expecting to be optimal with the Fd as I wanted lower spin, but in the end the 915F showed the most optimal performance numbers with head and shaft combination.