I am, for one, not all that articulate. In conversation I often find myself struggling to find the appropriate words, and I often feel quite inadequate in capturing my thoughts in real time. I think this has to do with the, in general, fast pacing and the spontaneity of conversation, and in it involving many factors besides the communication of ideas. For one example, we all have to be aware of implicit rules for when it is proper to answer, or interrupt the other speaker. These rules, or conventions, are context dependent and vary with persons and cultures etc. Some are more naturally able to pick up on those, avoiding having to deal with it creeping into conscious awareness clogging up the flow of thoughts. I´m not one of them. In addition to those, there are a host of emotional signals that others give off that I react to, that need not bear any direct relation to whatever it is that is the topic at hand. When I notice them, I can focus in on them to such an extent that it disrupts my ability to develop a longer idea. This leads me to the second point.
My thoughts on some matter are often far too intricate and multi-layered for me to be able to deliver them concisely and clearly within the time frame of an ordinary conversation where the average speaker (from my experience in my whereabouts) has about 20 seconds at most to deliver his, or her point. It requires a particular skill set, especially for someone that has views that differs radically from the cultural norm, to be able to do so effectively. I am sadly lacking in those. At worst the result then becomes a word salad, often, a sentence stopped midway for others to fill in, at best, a lovely fragment.
(For those interested, the philosophers mail has an interesting article about La Rochefocauld where you can read about, among other things, how he developed the aphorism to be able to cut through in the salons where they had no tolerance for 'leadenness or pomposity'. And lets face it, going on for 30 seconds+ about some topic that borders on the esoteric making sure every detail gets in will often have that character unless you are in a specific social context where it is expected, or at least accepted.)
When it comes to writing, however, I tend to do much better. For one, I can write when I feel like it. That means that I don´t have to force some thought that fits with the direction the conversation is going in. Instead I can write down the thoughts that are most pressing to me. Thoughts that I rarely find the social space for. And best of all, I can do so at my own pace which means that I have time to find the perfect (or as close to it as I can manage) word, or phrase for what it is that I want to say, and I can go back and edit what I have written so that I can create more complete, and accurate pictures.
To answer your question. I think that having an ardent desire to make a true representation of ones own thoughts, while having a rich inner world striving for a nuanced view on life in all it's complexity, will have the natural consequence of a bigger vocabulary.