The SQL Server documentation for sp_updatestats states that:
If 'resample' is not specified, sp_updatestats updates statistics by using the default sampling.
How exactly does SQL Server calculate the default sample size?
That's a good question. If you're interested in statistics, you might want to give Elisabeth Redei's excellent blog post a read:
It explains that, as the table size increases, the default sampling rate becomes insufficient to accurately represent the table's contents. It also provides a formula that she states is used in calculating the default sample set size (Rowmodctr 500 + 20% of total rows).