After writing first 100 values for a column (for 100 rows), the Parquet writer checks if this 100-values column content exceeds the specified page size (default is 1 MB).
If the raw data size for the column does not exceed the page size threshold then the next page size check is constantly adjusted based on the actual column size, so it neither checked after every column value nor after every 100 values. Thus the page size is not the strict limit.
If the raw data size size exceeds the page size, the column content is compressed (if a compression is specified for the Parquet file), and flushed into the Page store for the column.
This is a nice explanation of the process.