Was the file also created in Solidwoks 2011??? Have you tried to make a copy of it and then rename it and then make a save under to see if it's the same???
To understand this phenomenon, we have to go back to the origins of Solidworks, namely its 3D modeler, which is none other than Parasolid. And the phenomenon of "overweight" of Solidworks files must be found in other software using this same modeler, such as SolidEdge or Topsolid, but there are others.
Indeed, when you carefully read the docs extolling the merits of Parasolid, you can note that the 3D geometry is the result of Boolean operations (and yes, it's an old software, Parasolid. It dates from the end of the 80s, the time when the dinosaurs Euclid, Cadds, Catia v3 reigned supreme on the planet MCAD).
Extract: Functionality Parasolid advantage Modeling operators Powerful hybrid modeling operators enable users to create and edit new models by uniting, subtracting or intersecting any combination of wire, sheet and solid bodies. The suite of boolean-based operations includes: • Versatile region selection and matching options for optimized feature attachment • Instancing for fast replication of features • Generalized Booleans for operations involving disjoint, non-manifold or mixed-dimensional models.
Translated into French, this means that when I drill a hole, I subtract a cylinder from my volume, and when I think I destroy it in the function tree, well Parasolid does not erase it but fills it back... Hence the constant increase in file size.
This phenomenon does not exist with software like Pro/ENGINEER because it is not based on a Boolean modeler, but "feature based". It stores the history of the creation of functions, which means that when a function is destroyed, the size of the file decreases to the size it had before the creation of the destroyed function...
To explain the decrease in file size when deleting features in the build tree or when creating an "engulfing" cube, it must be taken into account that all exact solid CAD software represents volumes by their boundaries (Boundary Representation = Brep). And the more complex this geometry is (rounded, left surfaces, etc.), the larger the file size will be. By deleting or hiding the functions, we reduce the file size...
CQFD
by re-registering under another name often the size decreases once the part is finished as specified @ AcCobra
On one of our parts recently we had the same problem it was 120MB while other parts almost identical with the same number of lines in the family of parts was 0.5MB.
To regain a normal weight, we remove the family of parts and then recreate it and everything is back to normal.
If you can make a backup of your part you can try deleting and recreating this part family to see.
I did the test and it seems that sw has modified the principle, a cube without a hole was 155648 bytes before drilling, 184320 bytes after drilling and returns to 155648 after removing the drilling. In 2014 version