It's a result of search improvements.
Every record is indexed for search engine. You can select which tables are indexed - however it's since 1.6.1. Those without index won't be searched.
Indexing is a pretty complex issue.
1. Get what should be displayed and use 3-letter tokens - for Ladis it will be: lad, adi, dis.
2. Mark that certain token occurs in certain records - that's the role of recordbrowser_words_map table. Place in the system is 1. table, 2. record, 3. field, 4. position in field.
It's a huge improvement in search, but it's drawback is a space used for this. I'm not sure, but in 1.6.1 it's been improved to take less space.
In our case: 170MB overall, half of that is the index size (phpmyadmin shows statistics under structure tab), ~1,907,492 records in map table.
You don't have to backup this table, because all records will be reindexed with cron or user requests (since 1.6.2).