Why is a metal piece heated up when cathode rays strike it?
When cathode rays strike a metal plate, their kinetic energy is transformed into heat energy, which increases the temperature of the plate.
Why specific charge for positive rays much smaller that that of cathode rays?
Specific charge is the ratio of a particle's charge to its mass.
Since, the mass of an electron is about 1/1837 times that of the mass of a proton. Its mass is much smaller than the proton.
A positive ray contains alpha particles (doubly ionized He nucleus). Its mass is much larger than that of an electron so, the greater electronic charge has negligible effect as compared to its mass, and the specific charge is much smaller than that of the electrons.