How Big Will a HTPC Case Be in Comparison to Xbox One/PlayStation 4?

I'm looking at making a steam machine instead of buying a next gen console. I already have most of parts including the mobo which is a matx.

So because of the matx the smallest cases (that I can find) are HTPC cases but I'm just wondering how big in size they are compared with the new consoles.

Please suggest any alternatives if you have them. 

Thanks for any help.

Actually have something like this in the works. Here's the link:

Just ignore the two HDD's (they are spares I had lying around).

CPU will be the new A8-7600 Kaveri APU, (when ever AMD finally releases it to market) which will be run in dual-graphics with the low profile R7-240 GPU for some decent gaming performance. 

That case, the Inwin BP655 is the smallest with a half decent built-in PSU I could find. 

For mATX, you could go with the Apex DM-387 which is a nice looking, low-cost slim HTPC case. I'm not sure how good the PSU's in those are though. 

As for size comparison to the PS4 and Xbox one, the Inwin BP655 is pretty darn small. Probably close to that of the PS4 and smaller than the Xbox one for sure. The Apex DM-387 is slim but will be wider deeper than the others.