Number of verts doubles after using symmetrize tool in Sculpt Mode this is the first time I sculpt anything and that's why it is really confusing.
In the beginning I had 700.000 verts, file size 50mb
 

Then it got to 8.000.000 verts, 250mb, I didn't notice that


Now, when I edit the last model just a little bit, number of verts goes to 17.000.000, 400mb, after using the symmetrize tool

Stackexchange doesn't let me post the 3d image and deletes my "hello"

I haven't used any modifiers, exept mirror on eyes, added only a lamp. 
Although, at some point I colored some vertices.
Is this normal? If not, what could be the problem?
Never mind, just used "remove doubles" and its back to 250.000, and it no longer doubles verts