The default size of the user database is based on system database model.
When we create a new user database it is modeled after model database and created with same settings and properties. If your model database has large mdf or ldf file the same can be automatically replicated when a new user database is created. This was the actual case in the scenario mentioned earlier. On a separate note, with the default installation of SQL Server the mdf file of model database is 3 MB and ldf is about 1 MB.