Black spots on a phone display are normally because of pixels or the screen got damaged inside or there are pressure marks on it. For example on the Apple iPhone or other smartphones a black spot usually means some pixels have stopped working. This can happen if you drop the phone by accident or if you put much pressure on the screen like when you sit on the phone or if there is a problem with the hardware inside. Sometimes if water gets inside the phone it can also make dark patches appear on the display.
If the black spots get bigger over time depends on what's causing them. If it is one dead pixel it will probably stay the same size.. If the screen is cracked inside or if there is water damage the black spot might get bigger as more pixels stop working.. If the pressure on the screen is what caused the problem it can get worse too especially if the layers of the display are coming apart inside.
The bad news is that black spots on display caused by pixels or damage, to the screen cannot be fixed by just using software. Turning the phone off and again will not make the problem go away. The only way to really fix it is to replace the screen. So if the spot is getting bigger you should get the phone checked soon as possible to stop the display from getting any worse.