Why were India and Pakistan sanctioned in 1998 for nuclear tests?

According to the Peaceful Nuclear Explosion Treaty, individual nuclear tests were banned beyond 150 kilotons, and group explosions were limited to 1500 kilotons.

In 1998, both Pakistan and Indian nuclear tests were 40 kilotons each.

So, why were India and Pakistan put under sanctions in 1998?

Would the decision have been same if there had been a test explosion of size 10-15 kilotons in 1998? Coz, this was the size of 1974's nuclear test of India, and India was not sanctioned in 1974.