I answered you in the GM forum first, because i didn't see this post, but have moved my reply here instead. The Wei Ya is not a good quality chassis, and is almost certainly having the problem i describe.
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Normally a chassis (the electronics) designed to drive a 14” CRT tube can drive a 19”/20” tube without a problem, but there are many exceptions. You won’t have any trouble changing from 19” to 21”, or at least not of the kind you’re seeing.
There are two main causes for your problem. The first is that you have a good brand of chassis but it's old, and the caps are drying out somewhere. This is not very likely outside a PC chassis, as you would normally see other problems with geometry as well.
The other cause is more probable. This is what i think is happening, but i'd be keen to hear from some of the experts. I am guessing you have a cheaper, newer universal chassis. These are all that is made now, you cannot get anything else new. They used to be a bit better, but the ones now are bad. Unfortunately there is no market for expensive chassis anymore. One problem with these cheaper chassis is that they normally have a very low beam current limit. This is to compensate for a lack of control in other areas, and it stops you burning your tube and giving yourself too many x-rays, among other things.
The beam current limit is the total power going across the screen grids (total current from cathode to anode) and represents the total power of the image you can produce, and it cannot get very high on these universal chassis. An image on a small area can be bright, but a complete picture cannot. There is no way to fix this, you can change the heater resistors, but it won't do much. If you play with the individual colour settings you can get the brightness changes to change colour as well, but either way you cannot get more power to the tube - any attempted changes in this area will start to distort the picture and will probably damage the chassis in other ways. The best you can do with one of these universal chassis is to find a high-quality tube that remains in good condition, but this may not help much.
Finding an old chassis that is compatible with your tube might be the best idea, and Nanao's are pretty much the best arcade chassis. Another option might be to RGB hack an old TV, as these are normally stronger and better than the universal chassis.