I knew others who preferred to expand quickly and were successful at that strategy as well, but I liked the option to do either and still win. The type of "turtling" that I find intolerable is what was featured in C&C: Generals (to name one) where you could build a strong base defence line with missle launchers to deter foes while building platforms to get tons of money and power plants to build whatever you want. This meant you never had to expand and you could have more resources than anyone else by staying put. That is what I want to avoid. Having the ability to expand a few sectors and then only continue expanding when you're economy has outgrown your current outpost is fine by my standards.
Different strategy games have varying degrees of "resource hunger," if I might coin an RTS concept, that they impose upon players.
Some games, like C&C Generals, are fairly relaxed about resource hunger. These games allow choices that are highly amenable to hardcore turtling. Such as allowing players to generate resources within a compact amount of space, and allow weapons like passive long-range artillery that are highly safe, inexpensive to wield (if not to construct), efficient, and difficult to deny.
Other games, like Total Annihilation, impose severe resource starvation that can only be sated by claiming as many resources as humanly possible, and you never have enough. Whenever possible these games always favor rapid expansion and conquest, and turtling on a small portion of the map is simply not an option. You will always lose against an enemy that actually expands.
I think the best way to design strategy games that involve resource acquisition is to give the player a virtually unlimited appetite for resources. However, actually acquiring those resources should be a fairly challenging endeavor because the enemy is going to harangue you at every turn, while you do the same to them. But, if you don't expand, the enemy will, and then you will be hopelessly outmatched. Thus, expansion to secure resources is absolutely vital for your survival. And so is disrupting the enemy's attempts to secure resources. Because if you don't do both of these things, the enemy will, and you will lose. This leads to immediate strategic decisions and immediate player interaction, and possibly even battle.
Now, to be clear, defensive play is still absolutely viable. However, the meaning of "defensive" isn't quite the same. Reckless expansion where you do not spend any resources on defensive units and structures gives you the greatest possible growth. However, this also means your critical resourcing facilities are undefended, leaving you highly vulnerable to being raided by the enemy.
Defensive play is slower expansion with additional precautions, such as defensive units and structures, or investing in a larger military in the event of a large enemy attack. However, spending resources defending in this way does leave you vulnerable to the possibility that the enemy will have expanded as aggressively as possible. Which you can then counter by using your units to go destroy those undefended facilities.
Basically, rather than thinking about defending a single base, in the context of expansion a defensive player is one who expands more slowly but with greater security.