It's fat. It's solid. I bought it so I could write while wearing gloves and it works really well for that purpose. There is a screw top to make sure it stays together and the pen doesn't dry out.
It comes pre-loaded with a fine tipped pen...just not sure it's a genuine Sharpie as it the ink doesn't stick on shiny surfaces and the barrel is plain black, no markings. The inner pen does pull out with a firm grip and a bit of a twist to and fro but a genuine Sharpie, at least the version I have, doesn't fit in as a replacement. Hopefully I'll find one that does as it's an expensive disposable pen if not.