Hello Swapnil
Both IS-IS and OSPF are link state protocols, and both use the same Dijkstra algorithm to compute the best path through the network. However, they do have a few differences in their architecture. You can read more about the differences here:
Specifically concerning scalability, IS-IS does not use as much bandwidth to function as OSPF and its algorithm is less CPU and memory intensive. As stated in the above link, “Given the same set of resources, IS-IS can support more routers in an area than OSPF.” This can be due in part to the fact that IS-IS doesn’t require a backbone area such as Area 0 like OSPF does, in the fact that IS-IS strictly functions at Layer 2, and that it has fewer extensions and optional features than OSPF.
I hope this has been helpful!
Laz