Platinum Carbon Ink Help

I've been using several Platinum Preppys with the supplied ink cartridges. I have two blue/blacks - one in fine and and one in medium, and two blacks - one in fine and one in medium. I decided to just stick with the ink cartridges supplied with the pens as it seems a simple way to start.

When I received the black pen in medium, instead of using the supplied black cartridge I decided to try Platinum's Carbon Ink cartridge. However, I noticed from the get-go that the line created by the medium pen with the Carbon ink was decidedly thicker than the blue/black pen in medium.

Is this a fault of the pen, or a quality of this ink? I will also state the Carbon ink seems a lot "wetter" than the black or blue/black inks, although I haven't tried it in a fine nib Preppy yet. Any suggestions or ideas are appreciated - thank you.

In the picture below, from top to bottom:

Platinum Preppy Fine with Blue Black Ink
Platinum Preppy Medium with Blue Black Ink
Platinum Preppy Fine with Black Ink
Platinum Preppy Medium with Carbon Ink

I don't use a medium nib, so I don't know, and never used the supplied cartridges that come with the Preppy. I do know my Preppy with Platinum Carbon Black seems to write thinner before running out of ink, and had clogging issues yesterday where the line became thinner than usual. From the image the Platinum Carbon Black doesn't seem all that much wider than the blue in medium nib. You might try writing the medium nib blue and medium nib Carbon Black side by side and see how they compare.

Different nibs, even though same "size" vary enough to be noticeable. Different inks will react differently with the different nibs too.

The Carbon series inks are pigmented and considered waterproof. The ink is designed to dry on top of the paper instead of needing to soak into the paper. It might look more bold because the paper isn't absorbing the ink as much. But, it's more than likely just the way that the ink and nib are reacting with each other.

Just make sure that you don't let pigmented inks dry up in your pen.

Thanks for all of the help everyone. I went ahead and cleaned out my medium Preppy (first time cleaning a fountain pen!) and then put just the regular Platinum "Black" ink in and I like the line thickness better. As it's just for classroom work I'm not so worried about waterproofness.
I think I'll try the Carbon ink in my fine point Preppy to see how I like that

Fountain pens are fun!