This struck me as a bit unusual. Most of the time, ὄχλος is translated as turba. I did a search with the STEP Bible, and while my numbers might not be entirely accurate, it seems that out of 171 occurrences of a form of ὄχλος, 155 of them are translated as a form of turba.
These instances in particular seem odd because "multitudo magna" and "multitudo maxima" are redundant; both mean something like "a great multitude." (Maxima is the superlative form of magna.) Perhaps this is intended to indicate the exceedingly great size of the crowd.
As I've been following along with the Daily Dose of Latin, I've been reading the New King James Version. There, both of these phrases are translated as "great multitude." In the ESV, they're both "large crowd," and in the NIV, "great crowd."
