It's not the $100,000 in salary over three years you didn't get in exchange for taking equity after a deal which made the founders rich if not wealthy. Significant equity aligns your outcomes and interests and gives you a seat at the table when liquidity events are going down.
Significance is more of a sliding ratio. 0.5% is significant if the founders each have 6%. 10% is significant if they have about 25%. There should be only one qualitative step between stakes, and that step should reflect the size of the company.
A final tell on significance is if the ownership structure is relevant to the discussion. Significant equity alters it in a...wait for it...significant way. It signifies that the offer is about more than a hiring decision. If the next programmer passing the interview process would get the same offer next week then the equity isn't significant. Gates and Allen wanted Ballmer, nobody else was going to get 6% with an option to 8 because there was nobody else.
Good luck.