JKR never answered that; in the books, interviews (I searched entire Accio Quote) or Pottermore for *Chamber of Secrets*. The most the books have is Ron/Harry's understanding that Hermione wrote the word "Pipes" meaning that Basilisk used the pipes to move around the castle.

However, "the only opening big enough was Myrtle's bathroom" isn't really a fact, it's merely the entrance to the Chamber.

The architecture of Hogwarts is shown to be variable (shifting staircases, for one; Room of Requirement for another example); so it's plausible that Salazar Slytherin, one of the 4 founders, configured the pipe system to be able to open up wherever needed - since he already "programmed" that Basilisk will travel through the pipes by creating an official Chamber entrance in the girl's bathroom. That's just my personal speculation, not canon.