What happen in a current circuit if I connect two wire of different size

If the gadge of the wire is bigger it offers more resistance to current. Overalll circuit impedance decreases. Therefore current increases.

what happen in a current circuit if i connect two wire of different size
That depends on the resistance of each wire and the load applied. Total circuit resistance=R , One wire= R1, Other wire=R2 Load=R3 --> Total resistance R= R1+R2+R3
With current being voltage divided by resistance : the lower the total R is, the higher the current will rise - if the power supply allows for that.
And with one common-to-all current passing through three resistors, the voltages over each of them will add up to the totally supplied voltage. Using a 12 Volt DC supply that means that a specific load will see for instance 0.4 volt over wire 1, 0.6 volt over wire 2, and therefore only 11 volt over the load itself.
:)

If the wire gauge is increaded(made larger) the wire is able to carry more current ---LESS resistance to the current not more. If you substitute a larger pipe for a smaller one, the total volume of material moved increases,, same principal