I also tend to feel sweaty during and after long flights. I'm not sure if there is any hard research showing that it is or is not more common. I would personally guess that it is, but I don't have any evidence to back it up.
However, it's certainly possible that certain people do sweat more. You say it's the case for you, and I feel that it is for me personally as well. Why is this the case for some people?
Long time in one position against seat
You're typically sitting in the same position for many hours. This means that there is little or no air circulation between your body and the seat. It also depends on the quality of the seat, but it's quite common for it to get clammy due to your backside being pressed against the seat for long periods of time.
Long time, no shower
Canton to Sydney, which is quite a long trip.
In this example, you may be traveling for 35 hours or more. In that case, there is a simple answer: You're going for quite a long time without showering. (It's, of course, possible to shower during the layover or even on the plane, but most people don't.)
Normally, most people shower/wash themselves at least once a day. Some people do it twice a day. There are good reasons for that. Sweat, salts, etc. accumulate on the body, causing various uncomfortable sensations for oneself as well as others. Long air journeys can take much longer than that, sometimes causing you to not shower for > 50 hours. Perhaps, if you went that long without a shower unrelated to flying, you would notice some buildup of sweat too.
This may not be something you (OP) personally suffer from, but many travelers do. Cold sweating is one of the symptoms of airsickness.
I don't know if this applies to you, but a lot of people can get slightly stressed while going on long journeys. They have to pack everything, remember everything, perhaps get up in the middle of the night, have their documents in order, keep track of their passport and money, be on time for everything, pass security, customs, immigration, guard against theft, wait in lines, experience lack of sleep, etc. Most people (including me, certainly), can feel mildly stressed due to all this. It is a well known fact that stress increases perspiration.
Lack of sleep
Even if you are a seasoned traveler who don't feel stressed at all, and even if you've managed to shower during your journey, it's likely that your journey interferes with your sleeping patterns. You might get some sleep during your journey, but it's not going to be the same quality that you would normally get. Lack of sleep may possibly cause increased perspiration. Your body is tired, and has to work harder to perform its normal functions.
Here's a final point that I expect to be (and welcome being) challenged on, but I'm including it anyway: The air in airplane cabins is extremely dry. This may cause the body to compensate by sweating, etc. Of course, water also evaporates much faster in the dry air. However, only the water evaporates. The salts and other components of your sweat all remain on your skin/clothes. This may increase the feeling of being sweaty. (Again, I do feel that this last point is somewhat speculative, so I'm including it more as a hypothesis than a fact.)
Shower right before departing. If possible, shower during layovers. As @pnuts said, wear antiperspirant. Try to relax as much as possible during your journey, and try not to be in a hurry for or very worried about anything. Wear clothing that breathes well. Get up from your seat regularly and stretch a little bit. Bring extra sets of underwear/shirts/other clothing and change in the bathroom when you feel sweaty. You may even bring body wipes to clean your skin and reapply antiperspirant on clean skin. Do whatever you can to get enough sleep. If you can afford it, get business class which is much easier to sleep in. If you're rich enough, and it's available for your route, consider flight tickets that give you access to an in-flight shower.
Personally, I don't bother with all of this (although I certainly do some of it). I'm just trying to answer the question on how to counteract excessive sweating, though. To which lengths you may want to go would depend on how important this is to you personally.