There are a number of problems associated with a high number of lines of code:

• It can be difficult to understand and maintain, even with good tool support.
• It increases the likelihood of multiple developers needing to work on the same file at once, and it therefore increases the likelihood of merge conflicts.
• It may increase network traffic if you use a version control system that requires the whole file to be transmitted even for a tiny change.
• It may arise as a result of bundling many unrelated things into the same file, and so it can indicate weak code organization.