This means a more serious tax fiddle will attract a more serious penalty. The test for size is on two bases - the absolute size and relative size. But then the relative test kicks in. It asks how much the fiddling was in proportion to your overall business. Clearly the plumber is declaring only about half of his true income.
Gravity is really a test of how serious the tax evasion was.
For example, an accountant or other professional failing to declare income is regarded as especially serious. They are expected to know better. Also, if you have gone out of your way to evade tax it is more serious than forgetting to declare the takings which you took out of the till to buy coffee for the staff. Steps which make things more serious would include opening offshore bank accounts to take the hidden income, making up false receipts or arranging to buy some of your purchases in cash so as to hide the corresponding sales. Size and gravity are judged by the tax inspector at the end of the investigation.