With no PECL extensions or PEAR libraries to increase it's bloat, Kohana is extremely lightweight and massive libraries are avoided for more consistency and speed.
Kohana follows the OOP software paradigm and it strictly enforces the DRY principle, which makes it easier for developers to write clean and maintainable code.
Kohana has always been criticized for the poor documentation it has. Some things are not explained with details and some other things are not explained at all.