The answer is:

Metal N is the heaviest. When all the metals weigh 1kg, iron and M had a larger volume than N. M had the largest volume, so when it was cut down to 100cm³, it would be the lightest. The iron had a larger volume than N, so it would be lighter than N when cut down. Since N had the smallest volume when at 1kg, when it was cut down to 100cm³, it was the heaviest.

Interesting? I may be the only one who finds it interesting since I really really reallllllllllly like Science. :P