Not sure if I can get Zoroastrianism quite that large or Hinduism quite that small, but I can see how to make Zoroastrianism larger than Hinduism.
Basic PoD is a no Islam TL. Sassanids survive for a while longer, though they will eventually be displaced by another Zoroastrian dynasty. Their long wars with the Romans continue over the centuries.
Neither empire is capable of completely dominating the other, but eventually the post-Sassanids consolidate control over Mesopotamia, most of modern Syria, Palestine and Egypt. They are never able to penetrate further into North Africa than Egypt. Over the centuries, these regions consolidate as being mostly Zoroastrian, although Christian minorities remain, particularly in Egypt.
The Sassanids and their successors don't have lasting control over Arabia, but it falls into their cultural orbit. So do many of the peoples of the Caucasus and some of the steppe peoples beyond, though that region remains a contest for influence with the Romans.
To the northeast, Zoroastrianism continues to spread amongst the peoples of Central Asia. Buddhism is present in Central Asia too, but after winning an analogue to Talas, Zoroastrianism gradually becomes predominant in Central Asia, including Tibet, and much of modern Mongolia.
Within the Indian subcontinent and much of modern Afghanistan, though, the absence of Islam is a great boon for Buddhism. Northwestern India (and much of modern Pakistan and Afghanistan) was a particular stronghold of Buddhism in OTL, but was very strongly disrupted by the Islamic conquest, to the point where it largely vanished.
ITTL, Buddhism remains strong in northwestern India. Around AD 1000, a new Buddhist dynasty emerges there that conquers northern and central India, with more ephemeral control/cultural influence over southern India.
Under the rule of this dynasty, and its successors, Buddhism remains the religion of rule. The remaining Buddhist communities in India (which had not yet vanished at this point) experience a resurgence. This leads to a gradual process of conversion; not instantaneous, but steady. By the late twentieth century Buddhism is predominant over most of India, in some cases having syncreticised with Hinduism, but still something that could reasonably be call Buddhist rather than Hindu. There are still plenty of Hindus around, but they are the minority.