This answer is based only on experience and instinct. Therefore it is highly subjective and begging to be "shot down in flames". Turbulence is usually a mixture of various "patches" of disturbed air, some rising, some falling and some swirling.
Assume that the small 'plane and the big 'plane have the same wing loading The very big one however has wings which are more likely to span across several of these patches, which would partially cancel each other out. The small aircraft would encounter such patches individually and thus feel the full effect of each one.
Goodness! You beat me to it. By the time that I had composed my reply, you had already come to the same conclusion.