If three friends are out having dinner, and one disappears when the check comes, the remaining two end up paying a disproportionate share of the bill. Whenever anything is out of proportion — either too large or too small — it's disproportionate.
If you live in New York or Los Angeles or another very expensive city, you probably spend a disproportionate amount of your income on housing — meaning you pay more than you'd expect. If you have a sweet deal somewhere and pay very little in rent, it's still disproportionate — only this time it's less than you'd expect.
