If true, the drawable's reported internal size will remain constant as the state changes; the size is the maximum of all of the states. If false, the size will vary based on the current state.

Must be a boolean value, either "true" or "false".

This may also be a reference to a resource (in the form "@[package:]type:name") or theme attribute (in the form "?[package:][type:]name") containing a value of this type.

[Android Documentation]