Here's a simplified drawing of an MOS transistor:
The blue patches are the Source and Drain. The orange band is the Gate. The rectangle under G that extends between S and D is the Channel. Its length is L and its width is W. When the transistor is ON, the channel acts as resistor connecting S and D.Here's a simplified explanation: the resistance of the channel is proportional to L/W. That is, if you double L the resistance doubles. If you make L shorter, the resistance decreases. Similarly, if you increase W the resistance decreases and if you reduce W the resistance increases.
If you have a fixed voltage between S and D, the current is inversely proportional to the channel resistance. This is from Ohm's law, which states that V = I*R or I = V/R. This means the current is proportional to W/L. If you increase W you get less resistance and more current. If you decrease L you get less resistance and more current.Hope this helps.
