'Major' merely means to pursue an academic major (an academic field chosen as an area of specialization). It is the most common way of expressing this idea; 'what are you majoring in?' may be the second question you hear from a new college acquaintance, right after 'what's your name?'
'What are you specializing in?' is possible but less likely in that situation. 'To specialize in' does not require tertiary matriculation. You can collect stamps and specialize in Monaco; you can become a mechanic and specialize in engine rebuilding.