File server databases like Access/JET MDB and ACCDB files need to be periodically compacted and repaired for optimal performance and to minimize database corruption. This also eliminates database bloat that can occur as data is added and deleted in the database. Here's a Microsoft Knowledgebase article that discusses some of the issues.
Microsoft Access databases can be manually compacted from the Access menu. Unfortunately, over the last few versions of Access, the location of this command has moved around driving Access many users and developers crazy. Read this paper for how performing this critical system administration task across all versions of Access: