The 'Size' column in our YouTube audience analysis is our estimate of the unique audience for a single video from the channel, reached within one month of its release. To obtain this estimate, we take a sample of videos published by the channel, normalize their cumulative audiences to estimate the unique audience of each sampled video within one month of release, then taking an average across the sample of normalized estimates.
The resulting audience size estimate can be much smaller or much bigger than the number of subscribers of the channel. The relationship between these two figures (the channel's subscriber base and the expected unique audience for a video published by the channel) is complex: some channels receive the majority of views from their subscribers, whereas other channels have content strategies that achieve the vast bulk of views from non-subscribers.