Smaller than small is buying what?
No-one really wanted a smaller Mac Pro, and even people who kind of did didn't need Apple to make such a huge reduction as they did (even half the size would have been a significant reduction).
Same kind of deal with the iMacs; I didn't really notice anyone complaining about the bulk of an iMac, but Apple made it much thinner anyway. Miniaturisation does have a certain wow factor if it can be pulled off.
To gut performance ... is that really going to help sales? To create bigger cooling headaches ... is that really going to help sales?
The Brix doesn't seem to suffer either of these issues; it still manages to be a great little performer in a tiny profile. The main trade-off it makes is really that it doesn't have an internal power supply; I can understand the Mac Mini remaining a bit larger to account for that, but with better and better power efficiency in all components, it's definitely possible to slim down.
That doesn't mean they will; it's just as possible they could go the Airport Extreme route and slim the Mini down horizontally but make it a bit taller, even if the volume remained the same it'd still be functionally smaller. Though personally I hate the new Airport Extreme design, I do prefer the "flat" profile of the current Mac Mini in terms of aesthetics.
Dumping ports doesn't make much sense.
While I don't personally want them to, there is definitely room to do so; a Mac Mini doesn't strictly need an Ethernet port when you can use a Thunderbolt adapter, especially since it has good built in WiFi. FireWire is bound to go, and there isn't really much reason for the Mac Mini to have an SD Card slot on the back (it's not very easy to use in that position). So if they need to trim I/O there's plenty of options that don't really cause the Mac Mini to lose anything.
Anyway, I want to say again; I'm not saying I expect the Mac Mini to be as small as the Brix Pro, I just expect it to have similar hardware, or is even possibly waiting for Broadwell. I firmly believe that if Apple thinks they can make the Mac Mini smaller then they'll do it, I just don't know how much smaller; it could be as simple as trimming down the current size by 10%, or it could be more major than that, I definitely think if they wanted to they could approach Brix Pro sizes, and even Brix Pro + room for an internal PSU is pretty damn small, and clearly is enough room for the hardware a Mac Mini needs. I don't know for sure, it's all speculation, but it just seems like they'd wait for so long if all we were getting was a spec-bump.