Event Listings: H1 is huge, H2, H3, H4 all same small size

I would like to make the post title on the listing page (https://romanroadlondon.com/events/) a little bigger.

When I edit the GD Archive Item page, I can see that the post title is H2 [gd_post_title tag=’h2′]

When I change it to H1 it become enormous. When I change it to H2 or H3 they are both the same, much smaller size.

Thanks

Tabitha