A Chinese idiom
Chinese: 一个头两个大
Pin-yin: yi2 ge4 tou2 liang3 ge4 da4
一: one
个: (Quantifier), entries
头: head
两个: two, a couple of. (两: Means twain here. Means tael when used as a quantifier, roughly equivalent to 38 grams.)
大: Big. Here means as big as something.
Rough trans: One head, as big as two.
Meaning: ...Err, have you had the feeling that when you study, work or simply think too much, you felt like your head all swollen and heavy? Like you had to hold it up or it'd fall off?
P.S. This is what I felt like after work on my paper for four hours straight...