设大长方体的底面长为a米,小长方体的底面长为b米,高为h米,则：
h×a^2=18/13
4a×(2/3)=4b
b=(2/3)a
h×b^2=h×(4/9)a^2=(4/9)×(18/13)=72/117(立方米)