Having several subprojects there are a some scenarios varying in usage extend.
Typically: working on the sources of some projects, and just using others as code with source inspection (or just decompiled code).
This can mean faster compilation, or more source control.
To work with such subprojects or modules a build infrastructure like maven allows for defining dependencies.
The advantage of many repositories is a larger decoupling (like in no longer using a library in a larger project), better engineering (preventing cyclic dependencies between modules).
The advantage of a large repository: you can still check-out single modules, and you specify a structure, hierarchy.
The disadvantage of many repositories is that over time the structure may change. Maintenance of a snapshot/tagged version at a point in time will become hairy.
The disadvantage of a large repository: the structure can become messy. You will keep obsolete libraries.
Conclusion: "It depends"
I like a modular and layered structure: independant libraries (like for vector graphics), dependant libraries (DAO objects) belonging to some applications, large applications.
Use something like maven, do not be afraid to throw away version history and refactor at large. Go for a big structure, but keep reusable small libraries (like for PDF) independant.
I doubt this will help much, one has to experience organizing large projects, splitting them up, doing some refactoring.