How to achieve big crumbs?

To my knowledge, it's all about a good balance of extensibility and elasticity. And that depends on a number of factors, for example hydration, flour protein content and quality and mixing time.

But in the end, big holes are just purely visual. It doesn't change the flavor, it's just for aesthetics. 

One technique is stretch and fold.  In gluten free it's similar, just more like fold and squish because hydration is higher.  More hydration is good too.

There's no silver bullet here..

- your starter has to be well active and used at the right time

- the temp in your kitchen has to be ideally in a good range to promote the levain development; but that will also depend on the type of bread you're making as some will need warmer than others

- the hydration and increased autolyse helps

- building enough gluten in the dough

- building enough structure in the dough..

I'd encourage you to consider looking up Trevor and his e-book on the subject.. it's called Open Crumb Mastery

Open crumb with a uniform and lacy structure is like the lockness monster of bread baking to me.. everyone knows when they see it, but no one knows how to find it.. ;)

I would also add that careful handling and shaping so that the dough is not degassed will also help.

Your bread looks great!  bigger holes just let the butter fall through when you make toast!