Try more "massive" planets instead
I don't know whether these folks are talking about "size" or mass -
Accretion is an observed process. Many meteorite specialists have relatively firm estimates for the meteorite flux in tons per year. (This number does not include mass increments due to comets.)
Assuming this constant, which is a huge assumption, as the earlier flux is likely to have been much larger, the numbers over millions and billions of years are large. I don't know how large or significatn in relation to total planet mass, but it really doesn't matter.
Arm-waving about "size" instead of mass is no solution.
ep