#Excel, 19,604 bytes

`=CHAR(64+MAX(COLUMN(),ROW()))`

Paste this formula in `A1`, then drag all over `A1:Z26`.

Byte count:

 1. The formula is 27 bytes.
 2. You need to copy it 26^2 times. 27*26*26=19604.
 3. I and others thought the score should be lower because you don't really need to type the formula again and again. I now think it's irrelevant - we count the size of the program, not the work spent writing it.
 4. For comparison - see [this 28,187 chars JS answer](http://codegolf.stackexchange.com/a/22547/3544) - obviously, someone generated it rather than typing all this, but it doesn't change its size.