The bulky base, the name says it... its bulky.. so it rather rip off a hydrogen thats easier to reach. In this case, the terminal hygrogens on Carbon 1 (primary), are much easier to rip, than the hydrogen from Carbon 3 (secondary).
Kaplan blue book page 363 (towards the bottom) says:
"Often there are two possible products. In such a case, the more substituted double bond is formed preferentially."
So I am assuming the above statement does not apply when dealing with a bulky base in which case the less substituted product ALWAYS results??