Not all ISTps are big and buff, but all ISTps want to seem intimidating to others, because they don't want people getting too close and understanding them (although their hidden agenda is to love). ISTPs who are completely reserved in their emotions rarely smile, but it doesn't mean they can't have fun. As is said on http://the16types.info/types-ISTP.php:
"...remains cold and unapproachable when he loves, and does not hurry to trust feelings of his somewhat frivolous dual (The Psychologist). He is very jealous and mistrustful: he is horrified that his emotions will be ridiculed..."

ISTps will let loose around a select number of people they trust and understand, and then they will smile and actually talk.

I have met female ISTPs before who are a high percent sensing. Very active, competitive, and reserved although they have emotional outbursts that get way out of hand (ISTPs can have a weak Fe just like INTPs). When they have these outbursts they can get very physically violent and throw things; do things they regret later that get them into trouble. Most of the ISTPs I have met and befriended are guys, though. They don't seem to enjoy being jerks, that just seems to be something with teenage guys in general...

Also, just because they are sensors does not mean they aren't interested in philosophy or deeper subjects, they are always good to have along for a reality check on crazy ideas. An ISTp friend of mine loves philosophy, he just approaches it from a practical perspective.

Note, ISTP and ISTp are used interchangeably in this post, however, all references to this type should be considered socionics references that I have pieced together from socionics descriptions & personal experience.