I'm currently writing a fantasy novel and it's my first piece of work. My chapters currently range from 500 to 3,350 words. Is this too much of a dramatic change? Should I go back and pad out some of the shorter chapters? Am I overthinking it, or is chapter length that important?
Chapter length is important. Consistency of chapter length is not.
A few ways to use chapter length:
Think of chapters as a pacing tool. All other factors being equal--which, of course, they never are--longer chapters tend to slow the pace, shorter chapters to increase the pace.
The length of a chapter can suggest (perhaps subtly) the importance of the events in the chapter. A longer chapter can give more weight to the events, a shorter chapter less weight.
These two effects often fight with each other.
Assuming there are no requirements for chapter length you are supposed to meet, I don't believe it matters. Some books maintain a steady chapter length of twenty or so pages (Harry Potter), but I've seen several with a page long chapter, followed by an extremely long one (Inheritance Cycle). Still yet others maintain short chapters throughout (Anne of Green Gables). In my experience, such ranges don't really mess with the pace/flow.
In my opinion, everything about a novel or other piece of writing must follow from its whole. If the lengths of your chapters follow narurally from the story.you are telling, then those are the lengths they must have. Word count is the wrong way to evaluate writing (unless you write specifically for a certain word count, as in academic or news writing). Let someone read what you wrote and tell you what might be too much or missing. Don't follow the advice of a word counter.
Is this too much of a dramatic change? Unless your book is to be serialized by chapter, it doesn't look like an issue.
Should I go back and pad out some of the shorter chapters? Not necessarily. If a read-through demonstrates that the flow is not how you want it to be, chapter length may be changed, although usually indirectly: you may erase some slow parts, or tighten up some other parts, or put in some dialogs, descriptions or previously removed scenes. As a rule of thumb, what happens on a given chapter and how it is portrayed is the driving force for chapter length. Unless your book is to be serialized, as I said earlier.
Am I over thinking it, or is chapter length that important? You may as well use the chance to get some work done. You can make chapter length important if you give it meaning within your writing.
As stated by @Dale Emery in his answer, chapter length can be used to great effect. I like @what's take on it: "Chapter 10: Paul died." is a very powerful effect, especially if you take care to make all of the other chapters long by comparison.
Whatever you do, do not pad out chapters. You want them to contain nothing superfluous.
Instead, look at their structure. Are the chapters logical units? Does it make sense to group these scenes together, rather than chop them in half, or group them with the next ones, or chop them in half and then group them with the ones before or after?
If you have chapters that have logical structure even though they are both short and long, go with it.
If you want more uniformity, I would look rather to split long chapters than pad short ones. And if splitting didn't work, consider merging the short ones before padding them.
I am also writing a fantasy book and have found that chapter length doesn't matter too much. A lot of the time, chapter length only puts stress on you as a writer. If you're trying to reach consistent chapters of, let's say 3,000 words, but you can only make it to 2,000, so be it! Chapter length paces the book and helps to emphasize the action. If your chapters are too long, it tends to drag on too much information and the reader gets lost or bored. If it's too short, then the events seem anticlimactic.
Just read through it and give it your best judgement. If you plan on publishing this, it would be wise to run it by an editor.