Lets say that your solute is completely soluble in 5 ml of ether. Then why use 2 x 50ml or 1 x 100ml of ether to extract it? Because the extraction procedures are not 100% efficient. Assuming the extraction protocol is 90% efficient then if you do it twice you can get 90% of 90% of the solute out. If you only do it once then you only get 90% of it out. The amout of ether is irrelevant, its a matter of efficiency.