Sure, smaller ships may not be useful for attacking colonies, but you have more than a "tiny window of opportunity" when it comes to outposts. Outposts don't generate population, so their defense doesn't change over time. The only time it changes is if you set up orbital defenses (minefields etc). Otherwise, outposts are easily captured with a small fleet of small ships. If someone has 5 colonies and 20 outposts, and you go capture their 20 outposts with small ships, they're not gonna be getting much income to do a lot about it while you work on building your bigger ships to kill their colonies.