Large preferences (plist) files Browsing through my recent Yosemite backup I noticed that two of the largest files were com.apple.sidebarlists.plist (78 MB) and com.apple.recentitems.plist (10 MB). Both files are located in ~/Library/Preferences.

Investigating com.apple.sidebarlists.plist yields that the following key holds a huge amount of data under the key: systemitems > VolumesList > Bookmark > data

Is it normal for plist files to become so large? What kind of data is this (icon previews?) and why are the files so large?
Property List (plist) files can grow to large sizes. Ten megabytes is large for preferences but not extreme.

Reasonable Size

Apple's developer documentation mentions that preferences sync'd with iCloud have an upper limit of 1 MB:


  The total amount of space available in your app’s key-value store, for a given user, is 1 MB. There is a per-key value size limit of 1 MB, and a maximum of 1024 keys.


Bookmarks

The sidebar lists and recent items content you mention are bookmarks. Bookmarks are encoded blobs of data used by OS X to locate a specific file or folder. A bookmark frequently contains more information than just a path; information such as security rights and means to locate the original file should it move or be renamed.