The 3D Printing Template adds >15,000 mostly hidden 3D Text edges to the model right from the start.
An ‘empty’ model using the 3D Printing Template is 1091.72KB
The same file with the 3D Printer Build Volume component deleted and purged is only 10.37KB
Like @Box says, the printer box you choose is nothing more than a visual guide.
That is, it will not control or limit the size of your model nor tell you the exact overall dimensions.
You might find it easier to work with the Default Template and use ThomThom’s elegant plugin.
It creates a box of guides the exact overall dimensions of the selected Group or Component.
Draw Boundingbox — Extension Warehouse
Use the Tape Measure to measure the guides it creates and then simply Edit > Delete Guides