You can now use minTileHeight & minVerticalPadding:

Theme(
      data: Theme.of(context).copyWith(
        listTileTheme: Theme.of(context).listTileTheme.copyWith(
              minTileHeight: 0,
              minVerticalPadding: 0,
            ),
      ),
      child: ExpansionTile(

Note: This can also be configured using ListTileTheme after the following PR is merged: https://github.com/flutter/flutter/pull/159053