Smaller discs are easier to spin at high speeds. That might be a limiting factor.

Bigger discs may need a bigger motor and a slightly more powerful laserto read. These are special application discs and are meant to storagelarge data and not for normal use.

look into holographic disks, they can hold way more info, though it will be a few years before they are mainstream, for the purposes you propose, i think the corporations could afford them if they wanted. i think the main reason for not using large disk storage, is that they want all their info accessable without searching a library of disks or waiting for a drive to read a massive dvd, so they use servers.

It is easier to deal with a cartridge of several standard discs rather than one large disc. The cost of a cartridge system is not a whole lot, compared to having to deal with large discs and their drives. Standard sized optical drives and media are commodity items practically.

disk are on the death bed. no investments will be made into new disk technology. the chip has won, your pc will be the size of a pencil sharpener within 5 years, not prediction but rather fact.

DVD's have definite, long time storage life compared to a chip ora magenetic medium. NASA would prefer to have back-ups on DVD of 1 TB than risk the storage on any chip or magnetic media.

You're assuming that we common people aren't going to find a use for a terabyte of portable data. People said the same thing about gigabytes and megabytes. We'll show them wrong every time.

You're also assuming that 12 inch discs are a useful medium. Remember laserdiscs?

And you're also assuming that NASA and businesses want their data in a material form that isn't guaranteed from disappearing in the hands of a criminal, spy, or idiot; or guaranteed from being rendered unreadable in mass because of one scratch.