It is generally advisable to boil as much of your total wort volume as possible, as boiling less wort makes the sugars much more dense in the solution (higher specific gravity) which has a negative impact on hop utilization. Also, scorching seems to be more likely if you are adding extract to a thick wort - it is just harder to get it to fully disolve in a smaller volume of liquid.
O, guid ale comes, and guid ale goes,
Guid ale gars me sell my hose,
Sell my hose, and pawn my shoon -
Guid ale keeps my heart aboon! -- Robert Burns