MAJOR scales are considered MAJOR because of the MAJOR 3rd (M3) interval that occurs between the TONIC and the MEDIANT scale degrees. For example, in the C MAJOR scale, there are 2 half-tone intervals between C and D and 2 half-tone intervals between D and E (2 + 2 = 4). And since 4 half-tone INTERVALS are equal to a MAJOR 3rd (M3), it is considered a MAJOR scale.
REMEMBER: Lower case Roman numerals (i, ii, iii, iv, v, vi, vii) denote minor chord quality and that UPPERCASE numbers (I, II, III, IV, V, VI, V) denote MAJOR chord quality. For now, try to focus on how the interval pattern relates to the type of scale.
The above graphic illustrates a critical point about intervals.
And that is: they’re a way of figuring out (and naming) how many whole and half-tones there are between any 2 notes. For example: going from C to D requires 2 half-steps with C# being skipped. Same thing goes for D to E, F to G and A to B.
Getting it yet?