Explain what will happen if mass of the mercury drop increases?

The question is not very clear if you are referring to a particular case or you want to know the general effect of increase in mass of a mercury drop. I will answer the more general question.

If the mass of a mercury drop were to increase, while keeping other physical parameters constant, this would also cause an increase in the density of mercury. Density of a substance is the ratio of its mass to its volume. Thus, an increase in mass, while keeping the volume constant, will increase the density of the substance.

An increase in the density of mercury will affect its various engineering applications. Mercury is commonly used to measure pressure using mercury barometers (devices in which a rise in the level of mercury in a tall column is measured as an indicator of pressure). An increase in mercury's density will mean that it will rise to lower height in the barometer and hence 1 atmosphere pressure will no longer be equal to 76 cm of mercury (it will, in fact, be less than 76 cm).

Similarly, more dense mercury will also be able to support more weight due to higher buoyancy. 

Hope this helps.