They could have said diameter instead of length.
The difference in size is 1¼ - ⅜.
Convert the mixed number to a improper fraction:
1¼ = 5/4
Give the fractions a common denominator. The least common denominator is 8
5/4 = 10/8
1¼ - ⅜ = 10/8 - ⅜ = (10-3)/8 = ⅞”
