Geometry tells us that B@rt's 4x20mm tritium vials have 251 mm^2 of phosphor surface area and 251 mm^3 of volume. Merkava's 3x23mm vials have 217mm^2 of surface area and 163mm^3 of volume.
All things equal, B@rt's vials SHOULD be brighter, but are they really? By how much? I'm not counting the surface area of the ends, the dimensions don't account for the glass thickness, and the tritium pressure may be different, so the above numbers are just an approximation. If anyone has access to both of these vials, please do a side by side pic to show their relative brightness.
I cant vouch for the merkava ones,(so not really helpful to the point of this thread ) but the b@rt ones are definitely bright, large step up from the 5.5mm or 8mm vials, from just working as a marker, to illuminating stuff around the vial a bit. I glued 4 blue ones in the bottom pocket of a glass bottle I converted into a change jar, and sealed them in with some GITD epoxy, and at night the bottom quarter of the jar is notably glowing from across the room, and not from the epoxy either.