The phrase "smaller proportion" is correct and can be used in written English.
For example, "A smaller proportion of the population has access to healthcare than before."
A far smaller proportion is recycled.
This is a smaller proportion than expected.
A slightly smaller proportion reported tightening lending to small companies.
A smaller proportion of revenue comes from corporate income taxes.
What's more, business pays a smaller proportion of US taxes than it once did.
For example, a smaller proportion of African-Americans have flexible hours.
A smaller proportion of people absconding at the border are being found.
Unemployment insurance spending is consequently a much smaller proportion of gross domestic product.
On the one hand, matchday income is becoming a smaller proportion of the overall pie.
Wells has categorized only half of them as impaired, a much smaller proportion than some rivals.
As budgets get tighter, many services will receive a smaller proportion of the money available.
