The debug log has a max size of 2MB.
Log entries for events that are necessary for processing the debug log don't get truncated and will always be part of the debug log, but other log information that appears between the start and end lines of these log entries is removed as part of log truncation. (Most notably, system.debug statements)
In order to provide the most pertinent information, debug logs are truncated starting with the oldest log entries. The newest log entries are always preserved. The debug log is truncated by 200 KBytes when it reaches its maximum size of 2 MB.
So, you will most likely need to pare down the amount of information that is being output via debug statements in order to be able to view the data that you're looking for in the log. (i.e. commenting debug statements you're not using, commenting debug statements that output large collections of data, etc.) If you have 2MB worth of data from items from the list below, you will probably need to test a smaller block of code to reduce the number of lines logged.
From the Docs:
Each debug log can only be 2 MB. Debug logs that are larger than 2 MB are reduced in size by removing older log lines, such as log lines for earlier System.debug statements. The log lines can be removed from any location, not just the start of the debug log.
These items will never be removed from the log: