The Ah rating of a battery is how much energy is in the battery, not the voltage. A car battery is like 45- 80Ah. 17Ah usually means it will put out 17amps continually for 1 hr without the voltage dropping. Some are /10hr rate than /hr. IE 1.7A for 10 hrs. So the only problem with a battery that big is the weight. The voltage will stay the same, and will work the same as a small one. The 17 Ah battery will be a lot bigger and weighs more than the 7Ah. A 3-8 Ah battery is all you need for your starter.
You need to reduce the lb , not the Ah so you arm will say AHHHHHHH
17Ah...... ? from a garden tractor ?