the actual file size of codeigniter is smaller then most full featured php frameworks.
despite its small file size codeigniter is completely self contained and does not rely in any way on any other frameworks,
or package managers. why is this relevant and important? because there are some php frameworks that are completely dependent on the symfony framework for some of their functionality. or they are dependent on package managers that may or may not be in active use a few years from now.
finally codeigniter is light weight in terms of the rules it imposes on you the developer. some people criticize this as encouraging or allowing bad practices. however in truth it makes it much faster for a new developer to get up and running with a new application. it also makes it easier to build new functionality and then refactor your code to the more 'proper' way of doing things.