Small doesn't mean non-existent. One of the few things even the most extreme anarcho-capitalists believe the government should do is protect it's people from a hostile takeover on behalf of a non-anarcho-capitalist nation.
In addition to that, you need to constantly evolve your delivery vehicles to overcome newly developed defense capabilities.
In even the most bare-bones defense scenario, the needed investment is fairly large, and it depends on how large of an investment adversaries make.