The phrase "proportional basis" means that something is distributed or calculated in direct proportion to something else. It implies that the distribution or calculation is fair and based on the relation between different factors. Full definition
In these systems, seats are not allocated on a proportional basis; rather, they are given to candidates who win a majority in single - member districts. (businessinsider.com)
On a proportional basis, the national debt expanded 36 % faster than the US economy (even if you include inflation). (businessinsider.com)
Across the Basin, on a proportional basis, the Commonwealth used more and carried over less of its available water for each of the five years between 2008 - 09 and 2012 - 13. (environment.gov.au)