Vijay it is pretty obvious due to the impact which is coming from a very high flow rate. A drilling bit is moved using rotation from surface typically for larger diameters the highest OD of the drill pipe that is utilized is 5.5 inches 21.9 lbs/ft in weight (6 5/8" is the max drill pipe size in our industry). Here in the middle east we use this size of drillpipe to drill 26 to be 36 inch boreholes. To drill this borehole, the drilling fluid pump rate goes up to 4000 gals per min or else you will not be able to clean the hole. The impact and the friction losses in the annulus between the inside of the openhole and outside of the drill string (whether it is drill pipe or drill collars) is going to be massive due to smaller annular clearance. Moreover, the deeper you go in depth the higher the hydrostatic head of the fluid and hence the overall impact and pressure loss in the form of annular pressure losses. I hope this answers your question - HAS

thank you very much sir,

Yes, with increase in bit dia., the thrust needs to be increased, as well, to overcome the hydrostatic pressure from the ground and to keep the bit face flushed tightly to the hole bottom.

Yes it is directly related to the drill dia. Increased dia increases the chisel edge length which contributes to the 60-70% of total thrust forces. Other axial forces also increases with increases of tangential force with increase of dia.