It’s really tough finding accurate statistics on unschooling, therefore it’s really tough to assess the growth rate.
My suspicion is that unschooling is growing (much) faster than you suspect. The homeschooling population in the US is currently between 2–3% of the school-age population (2.8% in my state). The growth rate of homeschooling in general is reported to be growing between 7–15% annually. It is therefore reasonable to predict that unschooling is growing at a similar rate.
In my view, the most significant point of the increase in the numbers of homeschooled students is that families are voting with their feet, and the message is that traditional schools are not meeting the needs of our children. Therefore, parents are walking away from a $12,000 per year subsidy (the average spent on a public school student), and are assuming the additional costs and time associated with homeschooling.
Homeschooling (including unschooling) provides schooling choices that are not available in the traditional school system.