Hi Neha thanks for reaching out! Unfortunately there is currently no expand/collapse functionality on our ERD shapes. However we would love your feedback on this! Would you mind adding your thoughts to this feature request form?

In the meantime- you could try using layers and hotspots to achieve this. A hotspot is a shape with an action assigned to it. You could place some of the larger shapes on another layer and create a hotspot as a button to toggle the visibility of that layer on and off. For a general overview of how layers work in Lucidchart please take a look at our Create Multi-Layered Diagrams article. This section contains more specific information on creating hotspot buttons.