Explain how comparing 645 and 738 is different from comparing 645 and 649.

I understand that you compare the numbers in terms of which one is bigger.

If you compare 645 and 738 you know "immediately" that the second one is bigger: they're of equal length and then you only need to compare the first digit to know for sure that the one with the bigger first digit is higher.

in the case of 645 and 649 you have to look at the whole number, because the first two digits are the same.