Mechanical Advantage of Gear

PRAD CHAKRABORTYIS IT POSSIBLE TO INCREASE THE MECHANICAL ADVANTAGE OF TWO MATING SPUR GEAR TRAIN BY NOT CHANGING THEIR SIZE AND GEOMETRY

You have to stop using all CAPS.
The mechanical advantage is the ratio of the number of teeth on the two. So you cannot change it unless this ratio is changed. The power transferred relates to the size of the gears. To transmit higher power bigger sized gears will be needed.

But sir regarding power,as we all know it is multiplication of torque and angular velocity,I think lower sized gears will transfer more power as they can rotate easily for same torque.

rajarshi vikram singhlower sized gears will transfer more power as they can rotate easily for same torque.

For the same angular velocity transferring more power increases the torque, which needs a larger dia shaft and gear.

See under spur and helical gear design section here:
<a href="https://books.google.co.in/books?id=yNHuleaCMOkC&pg=PA134&lpg=PA134&dq=Mechanical+gear+size+and+power&source=bl&ots=p02w3ocrW7&sig=hKJ_XbUuO6DB3NbA8_UEahlMMQQ&hl=en&sa=X&ved=0CEwQ6AEwCmoVChMI5sXC48j4xwIVlgmOCh30Nwnc#v=onepage&q=Mechanical%20gear%20size%20and%20power&f=false" target="_blank" rel="nofollow noopener noreferrer">Rules of Thumb for Mechanical Engineers - J. Edward Pope - Google Books</a>