There is nearly none. TeXnicCenter only creates two files for each project:

• <project>.tcp: This is the project file. It is very small, because it contains only information about the main file of the project and wheter the project requires BibTeX and/or MakeIndex. All the other information displayed in the structure view is generated by the TeXnicCenter parser from the main file.
• <project>.tps: This is the project session file, which stores information about the files, that where opened in the editor, last time the user worked at the project.

Both files are in INI-file format, so that they a readable/editable with a text editor.

Both files together will require less than 2kB in most cases, so there is no reason, not to use the project support.