What's the remaining 35 gallon in the 75 gallon heater? Air? Already hot water?

Assuming you start with full tanks of hot water in both cases and use 40 gallons (draining the 40 tank and having 35 left in the 75, and then fill both up with cold water, the 40 gallon tank will start heating cold water, but the 75 gallon tank will start with a luke warm mix.

Both heaters (ignoring differences in insulation, different volume/area ratios etc) should increase the temperature of their full volume at about the same rate (simple ratios 40 gallon/40 BTU vs 75 gallon/75 BTU. The 75 gallon tank will reach 'hot' first though since it starts halfway there already.

If air, and you're starting with equal temp water in both tanks (and you don't burn out something that should have been submerged in water in the 75 gallon tank by running it half full and can manage the air pressure) the 75 gallon tank has an almost 2x as powerful heater (75/40), so will be almost twice as fast in raising the temperature a given amount of degrees.

In heating their capacity amount of water, they will take equally long to raise the temperature 10 degrees (again ignoring the little/unknown stuff as insulation differences and that heat losses increases as water temperature goes up and that the bigger tank has an advantage in volume/area ratio for that)

Reservation - I'm assuming you by 40/75 BTU heater mean heating elements that will put 50/75 BTU of energy into the tank per hour.
(Over here we rate heaters in watts, ie a power unit, while BTU is an energy unit (power*time) )