Liberal arts does include math and science. It looks like you are erroneously using “liberal arts” to refer only to humanities and social studies.
Science and engineering majors do spend more time on school work, probably due to more lab courses (and design projects in engineering). However, some H/SS courses also have large projects, and visual and performing arts can be time consuming (studio and performance). Business is probably the lowest workload major.
http://economix.blogs.nytimes.com/2012/01/20/why-students-leave-the-engineering-track/
http://www.nber.org/papers/w15954.pdf (table 6 on page 32)