Typically, lead acid batteries of the kinds used in our applications
have amp hour ratings based on the load where it will take 20 hours to
discharge the battery. In higher rate applications, like running that
light bulb, amp-hour capacity performance is normally degraded. The
degradation may not scale linearly so comparisons between 2 different
packs may not work out using straight ratios. Capacity curves as well
as voltage curves for the batteries at various discharge rates should
be available on-line at the manufacturer's web site.

Other things to consider...

The charging requirement for the different capacity packs may vary. Are
the batteries each fully charged at the start of the test?

How consistent are the 2 bulbs you are using? Could 1 bulb be drawing
more current than the other?

Capacity does drop off with use. Capacity could also be adversly
affected by abuse such as deep discharge. Could the use (or abuse)
history alter the expected results?

Danny Brotto
LS-8-18 "P6"