On space, trains and the performance of masculinity

Those two behaviors seems similar on the surface, but are actually potentially very different messages.
I'd refer you to the book "What Every Body is Saying" by Joe Navarro, on body language.
"Notice how confident or high-status individuals will claim more territory with their arms than less-confident, lower-status persons." (In this section he's talking about reading arms specifically, but the same is true for taking up room with your body in general, as he notes on p. 102 describing "splay behavior.") "A dominate man, for example, may drape his arm around a chair to let everyone know that this is his domain... higher status individuals will usually claim as much territory as possible immediately upon sitting down, spreading their arms or their objects (briefcase, purse, papers) on the table." (p.120)
But compare that to later in the book where he says, "The use of objects is a sign that an individual wants distance, separation, and partial concealment, because he or she is being less open--which goes hand in hand with being uncomfortable or even deceitful." (p.213-214)

Both are basically ways of saying "Don't fuck with me" but in using your body it is an offensive posture, in using objects alone it is defensive. Men are more likely to take up the offensive posture, women the defensive. I remembered this part of the book in particular because I typically take up more space on places like city buses using my body posture in a more masculine way.

I think the privilege therein would be not noticing whether or not your masculinity is being questioned.

As a woman, it seems I can't do one thing or the other without either being a lesbian or a frivolous, pink-loving tart. When I was in the Army, I used to keep my hair short. It was great for me, I loved not being forced to try and keep stupid-long hair in regs. But every male soldier assumed I was a lesbian; we'd talk and share dirty jokes and I would just be one of the guys. Then I would mention my boyfriend, and suddenly I went from being "one of the guys" to being a thing that made them nervous, that made them posture and snarl at each other like lion cubs, that was simply there to be looked at and, hopefully, fucked. I was no longer a soldier, and I didn't have that option. From then on, if I let slip that I liked to lift weights or that I practiced martial arts from a young age, or (god forbid) they found out I was bisexual, then it was simply assumed that I was actually just a closeted lesbian. If I talked to them about my admiration for Coco Chanel or how much I enjoyed decorating my room in the barracks or getting pedicures, then they would roll their eyes at me, and I would automatically become weaker than I was, daintier than I had a right to be and still be a soldier, and altogether just a joke to them. And I wasn't allowed to be these two things, at the same time, and occasionally I would receive a bit of hostility for not fitting in to either of these two categories they had for me. It wasn't enough that I was a soldier; I became a representative of my entire sex, despite not being the only female in my platoon, company, battalion, etc.

It's a strange thing, to be constantly aware of how others are going to interpret your sex or your sexuality. And it's a nice thing when you don't have to pay attention.

I have to say I agree with you. While I see many men taking up excessive space on the subway by spreading their legs as they sit, etc, I find that they are usually pretty accommodating when someone actually does sit beside them. As such, I feel like this may be more symptomatic of an internalized, subconscious male privilege that allows men to shift their weight around in an unrestricted way rather than a purposeful claiming of space. In contrast, I see many women spread their bags and purses out over the surrounding seats which seems to be more of an intentional act to claim territory, especially since women are generally taught to take up as little space as possible by sitting with their legs crossed inward, keeping their bodies small, etc. I am not sure if I would contextualize this as a political act on behalf of women who refuse to take up as little space as possible; whether men or women do it, it is inconsiderate to other people. But I definitely think this issue is a bit more complex than men purposefully claiming masculine privilege or power.