When the Replicator Agent receives data for a string (NVARCHAR) column that is larger than the column’s current size, it will auto adjust the column’s size so that the data can be saved into that column. If the data is greater than 251 characters, this size is adjusted to the maximum size supported for the database (for example, MySQL is 4000).
The main reason for this is because even though ServiceNow may show a column as a maximum certain size, in many cases you can type more data than the limit. For example, if the short description field on the ticket table is set to a maximum size of 400 as shown in the first attached screenshot, you can actually enter more characters than this (see the second attached screenshot where each line has 160 characters).
So the agent increases the size to the max size supported by the database to ensure that data is entirely captured and not truncated.