The Gateway Arch is taller with 630 feet compared to the Space Needle which is 605 feet. The higher number is the taller or bigger one. Compare the two numbers:
630
605
6 is in common in the hundreds place so that's the same
In the tens place, 3 is bigger than 0. Since 3 is bigger than 0, anything to the right of the bigger number is now considered "not counting", so 630 is bigger.