Why DNA double helix have a uniform diameter?

NCERT Page No.81.

The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). 

Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. 

Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. 

This generates approximately uniform distance between the two strands of the helix