Also, Goku only met Piccolo-Boo, Buutenks, and Boohan. He never saw Super Boo in his base state until he formed for a moment inside of his own head. The ones he actually canonically fought were Fat and Chibi Boo. But with Fat Boo he was just stalling for time, and he was dead so SSj3 wasn't raping his energy supply as fast. His only serious fight with any of the Majins was the final fight with the final, original Boo. It makes sense he would envision Chibi Boo. Like in the RoSaT, Son Goku tells Son Gohan to imagine Cell is killing him/people he loved to awaken Super Saiyan. Gohan responds, saying he had never fought or even seen Cell so he couldn't. So Goku had him use Freeza instead.
To contrast, Goku toyed with Fat Boo to stall for time (Fat Boo is weaker than Chibi Boo as well), never even saw Evil Boo, saw Super Boo in his base state for only a short period of time before Vegeta tore Good Boo out of his body, and never fought Boohan, Buutenks, or Piccolo Boo. He saw them briefly, and then whooped Boohan's ass with ease as Vegetto. Since he was fused when he fought Boohan, Goku likely wouldn't use that as a measuring stick. His only real, serious fight with any of the Majins was against Chibi Boo, the most violent of them all that destroyed the world he loved and killed his children and friends.
Don't read so much into it. Chibi Boo is quite simply the most logical choice, it means little to nothing about his power relevant to the other Boo's.