Resizing the overlay/dyn

I’ve been trying to increase the size of the storage for installation of extra packages as it is too small at a reported 274Mb.
As far as I can tell eextra material goes into /dev/mmcblk0p3 though that doesn’t show up in the mnttab.
That partition is auto expanded on first boot and so it has plenty of spare capacity yet the root overlay appears capped at 274Mb and I cannot find how/where this configuration lives.
Does anyone know something about this?