In my experience, most fountain pen cartridges waste a lot of barrel space — both lengthwise and diameter-wise.

First of all, why?

Secondly, are there any OEM or after-market cart's or converters that are OVERSIZED to intentionally utilize as much of the interior barrel space as possible?

My guess would be that manufacturers want to make as few converters and cart sizes as possible. So, they will design a 'standard' cart or converter and make sure all pens can use it. So on some pens, this will be a tight fit, and on other pens, a waste of space.

For the most part, I am unbothered by this, as one can always switch to carts for better ink capacity. I am alarmed at pens that only take short internationals though. What a waste.

If you really are depressed by all the wasted space in the barrel, then maybe it's time to consider what you can do with a pen sac and cartridge, or an eyedropper conversion...

To some degree, you want a bit of dead space in the barrel to keep the air-space in the ink reservoir from heating, expanding, and overloading the feed. If you're REALLY opposed to the waste of space, there's always the possibility of eye-dropper conversion....

I think even short international size is plenty of ink for most applications. The size of today's converters and cartridges don't upset me. If they were the only option, it would be a different story.

I've always assumed the "waste" lengthwise was so you could store a spare cartridge in the pen. All the pens I have which use the short cartridge, have room for a spare in the barrel.

I'm not sure what you mean by "waste lengthwise". The design of the converter requires the plunger to be extended when full, so the length of the converters almost doubles.

Parker cartridges have always been pretty big, but also the international standard (“Pelikan”) version exists in double size:

http://www.pelikan.com/pulse/Pulsar/en_US....yStore.36433./5

Converters indeed waste much space on their mechanics, but what else can they do?

All my Pelikans have come with 2 cartridges. One in the pen, one in the barrel. (Narrow end to the back.) I always kept a spare in there and never worried about running out of ink, or about the short cartridge being lose and falling off or something.
I think that "extra length" is for storage of a short cartridge.
My Parkers always had long cartridges or converters with them.

Silke

The insulation explanation was the one that made the most sense to me. It goes back to why Parker and Sheaffer dropped the body reservoir mechanisms around 1950. Later the cartridge converter configuration did the same.

The International cartridge thing (partially applies to the Cross proprietary cartridge also, just there is no Cross long). Two shorts are the length of one long. The Converter is the length of the long, or two shorts.

The problem with ED conversions is the ink warms up from your hand and heats the air space in the barrel. The heated air forces the ink out. The Japanese saw the problem and corrected the problem with the regulator valve. In the West the engineers gave up on it and developed what they considered a better solution.

I have warm hands and I find the EDs do not work well for me. OTOH, I've yet to have blobbing problem with a cartridge.