I don't think context size is really the limit for larger codebases - it's more about how you use that context.

Claude Code makes some efforts to reduce context size, but at the end of the day is loading entire source files into context (then keeping them there until told to remove them, or context is compacted). One of the major wins is to run subagents for some tasks, that use their own context rather than loading more into CCs own context.

Cursor makes more efficient use of context by building a vector database of code chunks, then only loading matching chunks into context (I believe it does this for Composer/agentic use as well as for tab/autocomplete).

One of the more obvious ways to reduce context use in a larger multi-module codebase would be to take advantage of the split between small module definition (e.g. C++ .h files) and large module implementations (.cpp files). Generally you'd only need to load module interfaces/definitions into context if you are working on code that uses the module, and Cursor's chunked approach can reduce that further.

For whole codebase overview a language server can help locate things, and one could use the AI to itself generate shortish summaries/overviews of source files and the codebase and structure, similar to what a human developer might keep in their head, rather than repeatedly reading entire source files for code that isn't actually being modified.

It seems we're really in the early days of agentic coding tools, and they have a lot of room to get better and more efficient.