Q. Why the size of spare tyre is different from the pre-installed tyres?

There are significant manufacturing differences that make it clear that your spare tire is for temporary short-term use only. Your spare tire, be it a full-size non-matching spare tire or a compact spare, is typically a smaller diameter than your four normal use tires. It may be a slight half-inch up to a couple of inches in diameter difference and the width is usually significantly less than your factory tires. This is to save space in your vehicle when the spare tire is stored. The structure of the spare tire rim is less important than regular use rims, so it may have a much different wheel offset, or may even be made of lightweight steel compared to normal wheels. Being smaller and made of lightweight material makes it easier to install when necessary but also not usually as rigid or capable of the same abuse a normal wheel receives on the road.

It's just a cost cutting measure.