Network usage is strange when a topic is listen by multi DataReader

[ruoruoniao]

I met a very confused problem, I don't even know it is a bug or not...
When a topic has only one DataReader, everything work well, but when there is multi DataReader, network usage will be a very low rate.
In my first test program, if I use multi topic for four DataReader, it will be 400+ Mbs, but if I use single topic, it becomes to 5.x Mbs.

https://github.com/ruoruoniao/CycloneDDSConfused.git
First test program is in "First" branch.
Usage for one topic:
PingPongOneTopic.exe a b true
PingPongOneTopic.exe b c true
PingPongOneTopic.exe c a true
Usage for multi topic:
PingPong.exe a b true
PingPong.exe b c false
PingPong.exe c a false

Then I think it maybe some logic effect, so I make a simple HelloWorld program.
Also, when there is only one topic, the network usage will be very low.

https://github.com/ruoruoniao/CycloneDDSConfused.git
HelloWorld program is in "main" branch.
Usage for one topic:
ddscxxHelloworldPublisher.exe
ddscxxHelloworldSubscriber.exe
Usage for multi topic:
ddscxxHelloworldPublisher.exe a b
ddscxxHelloworldSubscriber.exe a
ddscxxHelloworldSubscriber.exe b

So what reason cause this? How can I do to improve this?