Anything can be offensive, or not. Offense is in the mind of the subject, and may take intent of the speaker into account.
In my particular culture (Western Canadian Anglophone Caucasian, which overlaps with lots of other cultures, especially throughout North America), at this particular time (2010's, but extending back for quite a few years), I would expect a significant number of women to take offense to being called "chubby", even if it was not intended to offend, and especially if they did not have a very familiar relationship with the speaker.
For what it's worth, most adjectives implying overweight would also be considered offensive to varying degrees. Being overweight is one of the most sensitive personal issues (especially for women) in my current culture & time. In fact "overweight" is probably one of the terms least taken in offense -- which is why I'm using it.