Some Spotlight metadata importers (which we use to extract indexable text from documents) only return the first 10 MB of the file content. It is the case for the plain text (.txt) importer.
We will add a hidden preference to FoxTrot 7.1 (that can be set from Terminal.app) so you can extend this limit for .txt and .log files.
To extend the limit to 1 GB:
defaults write com.ctmdev.FoxTrot PlainTextFileLimitMB -int 1024
To completely remove the limit:
defaults write com.ctmdev.FoxTrot PlainTextFileLimitMB -int 0
Note that, in case you have large non-textual files with a .txt filename extension (or TEXT filesystem type attribute), this could considerably degrade the performance of the indexer.