Unlike P2/DVCProHD where data rate is tied to the frame, XDCAM data rate is based on time. 1 second of video, regardless of frame size or rate is 35mbits VBR. There some ability for it to vary but basically the data rate is the same. In HDV compatible mode it's 25bits CBR hence you get longer record times.
So P2/DVCProHD uses much less data when recording 720p24 compared to 720p60 or 1080 for example. This is NOT the case with XDCAM. Personally I'm glad XDCAM does NOT work that way. I can then choose my recording mode best on what's best for the job rather than trying to extend record time to the a card. While it's true that at 1080 you're spreading that data over more pixels on a frame compared to 720 and the data for a given frame will be different for 24 vs 60 as well but I find those choices more reasonable then limiting myself to 720p24 to try to increase record time.