They are getting larger. You're looking too far back. After the Great Depression, policies were enacted which served as dampers to improve system stability. They worked for quite a while. But eventually these policies were eroded or repealed, and other societal factors came along which degraded stability, so the oscillations picked up again, and grew larger and larger (think 80s S&L crisis, '00 dot-com crash), culminating in the '08 recession. Now we're going to have a much bigger one with Trump and the GOP in office. If this one doesn't cause a collapse, the next one will.
The dot com crash was big for the stock market and for the tech industry, but it wasn't a really big recession; it barely touched economic fundamentals, just look at GDP.
Personally I doubt the next recession/slowdown will be anywhere near as big as 08, but I guess we'll see.
We always have recessions when there's Republicans in power (I'm not sure why voters haven't figured this one out yet); there was one with Reagan, one with Bush I, one with Bush II, etc. Trump and the current crop of Republicans are advocating very extreme changes (namely trade restrictions), so I think we're likely to have a more extreme recession because of this.
