@navid
If you are comparing two pass transistors in an LDO, the Vds is the same since it is the Vsupply - Vregulated.
The gate voltage would be different depending on the current through the pass transistor.
If the transistor is not sized large enough, the gate voltage would be such that the pass transistor would be in linear region, else if it is large enough then it would be in saturation.
And kishore680 is right that with large transistors, when the load current is low, the transistor would be in subthreshold which is ok too.
As long as your loop gain is high enough you would have the required load/line regulation. So just keep the Amplifier/Driver stage strong.