Although I am a huge fan of tanks, I don't think this can be done, without reducing the crew. As far as I remember, we like having more men in a tank - it adds redundancy and reduces stress on each crew member.
....perhaps a day will come when tanks are made of carbon/ Kevelar composites with reactive armour surounding them.....who knows, stranger things have been developed by the Military / industrial complex!
You've probably put your finger on the basic engineering/human factors design problem. I suppose part of the problem is defining the mission, and putting the right number of guys into the vehicle. (This story makes it sound like you'd need no fewer than 3.)
If I read these guys' comments correctly, the primary requirements are firepower and armor, with speed being a second-tier requirement.
It also sounds like there's a serious need for close-in, anti-personnel armament ("we were fighting out of the hatches with rifles and pistols...") They also need some heavier stuff (probably not an M-1 sized gun, though) -- I don't know what sorts of "short, big guns" might work. Sounds to me like really big firepower needs could be met by a combined-arms tactical solution, rather than trying to put it all in one vehicle.
Just a thought. How about an Urban combat version that eliminated the heavy gun and its heavy large ammo, saving a lot of space and weight, and replaced it with multiple 50 cal machine guns which could be fought from inside the tank (instead of standing exposed in hatches and fighting).