1. Not necessarily, some animals may need to be smaller to survive, and thus evolve smaller.
2. Evolution does not occur between bodily systems, just how much of each is needed. Bigger animals need much thicker bones and larger muscles, and usually also have thick skin, but they don't have to give up one to get more of another.
3. Where did it come from? Nutrition and food you fucking idiot!
Crazy analogy:
One of the major computer building problems is getting a lot of RAM, you want a computer that has a lot of RAM, a big enough power supply to run everything, and a chassis to hold it all.
If so, where does the extra power supply, RAM, chassis come from?
If RAM evolved into chassis, the computer would be too low on memory to run anything.
If power supply evolved into extra chassis, the computer couldn't get enough power to run. So, where does all the stuff come from?