I may not have expressed my position clearly. Because most costs of living are fixed and not proportionate to income, taking 20% of the income of a poor person will have a disproportionate affect on them, as opposed to someone with a very high income for whom the difference will only marginally affect their standard of living. Take 30% of someone's $20,000 income and you will deeply compromise their ability to exist comfortably in society. Taking 30% of someone's $5,000,000 income will have no effect on their ability to cover their needs and live comfortably.

A flat tax, then, is actually quite regressive, rather than neutral.

Speaking in terms of companies, though, there is also a question of proportion vis a vis size—bigger companies and their employees tend to cost taxpayers quite a bit more than small businesses. There are no giant Twitter-like subsidies for five-man firms, for example. Larger firms are much more likely to make use of legal infrastructure, to need regulation, and to have the power to seek accommodation by the government for infrastructure needed for their given business.

In common English, rather than mathematics, proportionate merely implies that these things are related in size or amount. There is no implication of a fixed ratio. Replace "proportionate" with "appropriately related" if that helps.