The nice wizards says
If this is a large table or view, or if the chosen full-text catalog has indexed or other large tables, consider assigning this table or view its own full-text catalog
"Large" is a pretty subjective term, is it 1000 records or 1,000,000 in a table? What guidelines do you follow when creating additional full-text catalogs? Is it worse to have 1 full-text catalog handling many tables, or a 1-1 relationship between catalog and table? I've usually only broken them up based on specific rules, e.g. this catalog ignores any non-alphanumeric, this one allows everything, this one is accent sensitive, etc.
Have a read at this FT white paper published by the SQL CAT team: SQL Server 2005 Full-Text Queries on Large Catalogs: Lessons Learned. It is more broad than your specific question, but covers the subject of FT scale and performance with some good hints.