Unless the J-20 designers were forward thinking and designed the hardware that the rails clamp onto to be variably adjustable in a lateral fashion, so that future rails could be slid over to make room for a third rail in each half of the weapons bay.
At this stage, we are still in flight test, and to be honest the J-20 has been designed to be more about "stealth" and less about carrying a bunch of "odd" weapons just to be classified "multi-role"? Stealth aircraft of necessity have smaller bays than many of us would like, and it may be true the J-20 bays are "deeper".
In this case the airframe has been designed to have a very low RCS. That continues to drive this test program, the result is a smaller weapons bay, likely removable hard points that depart the aircraft after weapons deployment.
I would be more concerned at this stage about the actual performance of the aircraft, and how it will function operationally, the side mounted weapons bays mirror the F-22, and the J-20s main weapons bay is very similar in size, so it will no doubt be functionally adequate, without taking away space from critical aircraft systems.
The weapons bay is "right sized" to the J-20 airframe, and not vica-versa, everything has to function as a whole, and then it has to "perform" up to a very high level, the engineers know what they are doing!