in bacterial plaques why are often different sizes of haloes seen?

Answer 1

I am unsure of the context of your question. It sounds like you are growing bacteria in petrie dishes and noting the sizes of the halo in the colony. This most commonly occurs in the lab setting where antibiotics are placed on the bacterial culture and the sensitivity to the antibiotic is measured by the size of the halo - to do with how much bacteria it kills.

Other things causing halos on the petrie dishes would be bacteriophage which is a virus that can kill the bacteria or merely modify their DNA.

Answer 2

Acidity of the plaque