Personally, I am not a fan of big keys - for said reason regarding pockets. I suspect the Ferrari one is so big because hell, if I paid that much for a car I'd damn well want everyone else to know it too.
Now, personal opinion aside - to answer your question, the easiest/most cost effective way to create what you are describing is to do this: measure your current key, design a "shell" - two halves that snap together, that have a cavity to the shape of your key and will snap closed around it - draw this up (or have a friend with the requisite skills do it if you do not have them) in a CAD program or whatever software that can talk to a Makerbot/3D printer. Have same friend (or another friend, or buy your own) print the two halves out via their Makerbot/3D printer. Sand, smooth and prep the outsides of the halves - paint whatever color you want. If you want a logo, have this log integrated with the CAD drawing prior to being printed out. Once you have done all the painting and stuff you want, simply snap it around the key. Be sure to design a small recess that you can fit a coin into to separate the halves once on the key - similar to how some remote key fobs do it.
Using this method, you can create all kinds of shapes and designs to snap around your key(s).
Edit: thinking more about this - if you have a friend with the equipment, you can get this made for pretty cheap. Even if you "hire" a shop (assuming they are willing to do a small one-off job) it should still be pretty cheap. The most time-intensive part will be in designing the halves to correct dimensions so your key doesn't wiggle, and to get a snap-together mechanism that will work. But you can model something like that off a remote key fob, like what I mentioned earlier.
