Slab Lens conjecture for one very small volume

Let $T^3$ be the flat cubic three-torus, and consider a double bubble with one very small volume and two moderate volumes. The Slab Lens is one of the candidate double-bubble types.

Slab Lens conjecture. For one very small volume and two moderate volumes, the Slab Lens is optimal.

This is presented as an intuitive conjecture suggested by the computed phase diagram; the source gives no proof of it.