In/o/vations

do big small big small instead

The compression ratio on the smaller cylinders wouldn't be high enough to burn fuel in them.

why not just inject less fuel into the smaller cylinders?

Variable displacement
themotorreport.com.au/car-review/size-does-matter-hyundai-imagines-an-engine-with-different-size-cylinders-86448.html
autoblog.com/2016/05/09/honda-patents-engine-different-cylinder-displacements/

If you reduce the bore by half and keep the stroke the same, the compression ratio drops to like 25% of the original compression. You need compression to burn fuel efficiently. The smaller cylinders would just fill up with fuel.

Only if the combustion chamber is the same size as the original bore/stroke

why not inject just the right amount of fuel to achiever combustion at TDC each time?

what do you mean with 'it was balanced'?
You know the reciprocating mass force of all cylinders is best to be 0. As well as the torque opposing masses create.
A real WAT engine would be one big cylinder in the middle and two smaller ones on either side.
Primary vibrations and balance are ok. Secondary vibrations will still be present and power-delivery will be crazy with 4 phases being Big-small-zero-small-.

you need COMPRESSION.

You sound like a non-car person, so I'll try to put it as simply as possible.

Balancing issues. It's hard to balance stuff which isn't equal in size. You'd have to add a shitton of ballast to the smaller pistons to have equal weight for each cylinder -- assuming that you still have the same conrods, and pins, and rings, and this, and that.... It's too much work.

Also, production. It would be far easier to have four balanced pistons that are identical to each other. Why have two production lines producing two different types of pistons and their associated accessories, when you can do with just one production line of identical components?

If you want to change the displacement of the engine to something smaller, then have smaller overall pistons, or even less crankshaft stroke. You can even go for less pistons (3 instead of 4) instead of changing each cylinder's shape to something smaller.