Why does the amount of adipose tissue contribute the most to the total body water amount in humans?
Fat mass does not take up water.  It brings with it less water percentage than lean mass and so a person with more fat mass will have relatively less % body water.  Additional mass as fat contributed relatively less water than additional mass as lean tissue.

Consider a 50 kg individual who is extremely lean.  From your numbers, if this individual is 75% water (will will assume lean tissue is 75% water) that is 50 * .75 = 37.5kg water and 12.5kg nonwater body mass.

Now this 50 kg individual puts on 50 kg of fat.  The fat is 10% water (from your numbers) so an additional 10 kg of water.  This individual its now 47.5kg water and 52.5 nonwater; since this obese person weighs 100 kg now she has 47.5% water, similar to your example.  

Suppose instead your 50kg person grew up into a 100 kg performance athlete.  Still no fat.  She is still 75% water because the lean tissue has 75% water.

It has to do with the % water each type of body mass brings with it.  This is with no special knowledge; using just the numbers in your example which is what your professor wants you to use.