I also tend to feel sweaty after long flights. I'm not sure if there is any hard research showing that it is or is not more common, so I'll skip to the next part of your question which is why certain people (myself included) feel that way.

Long journeys

> Canton to Sydney, which is quite a long trip.

In this example, you're traveling for 48 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 at least once a day. Many people shower twice a day. There are good reasons for that. Sweat, salts, etc. accumulate on the body, causing various uncomfortable sensations. 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.

Mild stress

I don't know if this applies to *you*, but a lot of people can get a slightly stressed while going on long journeys. They have to pack everything, remember everything, 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, lack of sleep, etc. Most people (including me, certainly), can feel mildly stressed due to this. It is a well known fact that [stress increases perspiration][2].

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 as you would normally get. Lack of sleep can [cause increased perspiration][3]. Your body is tired, and has to work harder to perform its normal functions.

Long time in one position against seat

Also, 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.

Airplane air

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.][4] 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 as a hypothesis more than a fact.)

Prevention

Shower right before departing. If possible, shower during layovers. (And if you're very rich, and it's possible, chose flight tickets that gives you [access to a shower][5].) Try to relax as much as possible during your journey, and try not to be in a hurry for anything. Wear clothing that breathes well. Get up from your seat regularly and stretch a little bit. As @pnuts said, wear deodorant. Do whatever you can to get enough sleep. If you can afford it, get business class which is much easier to sleep in.