Slack1_more - you need to take a very large chill pill. This is a simple question with a simple answer, maybe it could have been put better, but that could be down to the language barrier.

Short answer is down to what the disc creators put onto each disc. In Slackware and (I believe) Suse, you get the base structure, networking tools, desktop, lots of software, server setups and tools and so on. They are pretty complete sets and you could do an entire install completely offline (then, when your box is set up securely you can go online to grab updates). The smaller discs are set to make you go online immediately - Debian has a netinstall disc which has just enough to get your box started and the rest of the install is done online.

Firstly to say I was wrong on that point - a memory-stick copy of Knoppix 6.0.1 does include a "Copy to (another) flash device" option, I was not looking in the right place (Taskbar menu, "System tools"). Having said that, in Windows XP I've had surprise failures trying to copy a file from one USB flash memory device to another, presumably either the hardware or the software is unhappy when addressing two USB or flash devices in the same copy operations. For file transfers I can just copy from flash to hard disk then from hard disk to flash.

For the record, the operation also failed from USB-connected CD with a Maxell "retractable" 1 GB memory stick that comes with a utility called iStar that allows the stick to be partitioned into hard disk / zip disk / password-locked / bootable: Knoppix could mount the stick's partitions but couldn't put itself on the stick because, I think it said /dev/sdb1 didn't exist. Instead of debugging this, I bought a Sandisk stick to use instead (2 GB): that worked fine - leaving the volume format as Windows FAT and including at least one hidden file: I don't know if otherwise it simply copies files from the CD.

This PC is a subnotebook with no internal optical drive, and it's a lot more convenient - if I don't want to put Linux on board that one's hard disk, and I don't, I need to run Windows things on it mostly - more convenient to use a small stick than a bulky fiddly external drive.

To return to the distribution question, there are other ways to pass a distro around besides downloading it yourself, but as it happens my main Internet connection is HSDPA cellphone network with a 3 gigabytes per calendar month quota, which is usually much more than I use but a full DVD is 50 days' allowance - which I can usually spare at the end of one month and the start of the next. But a CD ISO is more affordable. Most homes and businesses with modern computers can copy CDs and DVDs aqually well, but if you're just tasting or collecting different distros, CD-Rs are also cheaper. But then a rewriteable DVD is probably cheaper than coffee to go, so it isn't an expensive hobby just because of the disks.

Anyway, I'd say Linux doesn't have a solid line between the operating system itself and the unnecessary extras, but the files on a distro DVD that aren't on an equivalent CD tend strongly to be some unnecessary - but interesting and useful - extras.