I believe a command station will only ever support about 500 units (depending on your ship cap) so the AI will always opt to reinforce elsewhere once it reaches that threshold*. Those units will only attack you if you stray within their range, go for the comm station or due to border aggression, which will only ever be a fraction of the total force and fairly infrequently.
It's usually better not to incur the AI Progress unless you need to capture the system for strategic or tactical reasons, as at certain thresholds AI Progress increases will cause the AI to reinforce both more frequently and with higher Mk ships.
*Unless the AI can't reinforce anywhere at all, at which point it will start to build barracks and then carriers - but that's another story entirely!