Have you noticed this? I have.
No. There are several threads over the forums claiming this. Most times it’s user error, different volumes or inserts in control room, different settings for playback softwares etc.
Please be more specific about how you listen to the exports.
The Cubase playback engine seems to be a lot more forgiving of over-hot levels than when you bounce to an .mp3 or .wav if I recall, could this be a factor? It might make your mixes sound muddier when you play them back through iTunes or whatever. Also, different volumes makes a huge difference, I reckon there’s typically about 10-15db of gain difference between projects I’m working on and the music I listen to, but that might just say more about my mixing…!
It definitely can be my mistake also. But I have my master chain on, no plugins in control room section. Then bounce wav or mp3 file and listen it via windows media player and then compare that against cubase project playback.
My friend says that he also has spotted the difference with his own bounces with cubase.
You might be right. I have to do some more testing with this.
Import the mixdown file back into the project and a/b it there rather than in a different player.
Have you tried normalizing the .wav file after it’s created? That might help.
Audio/process/normalize
I have to try that.
I experience the same issue. The project mix of the .cpr file in Cubase sounds great. But every time I export to a wav file, the result sounds much different. I’ve tried many combinations and variations without success. Using Cubase 9 LE AI, and Audacity.
Guys, if that was true, I’d had to call myself an idiot. Even long term idiot 
Indeed reimporting the mix into the project allows for proper A/B’ing. That way the signal path from software through interface to speakers is identical. I think most differences simple happen in different signal pathes (does your player use the same channels of your interface, any EQs or so called sound improving algos in there, level @ 100%?).
Flip polarity of the reimported mixdown. You should get either silence or some residual noise from plugins with random (analog modelled) processing, modulation effects, stuff like that.
You need to make sure you don’t run the mixdown through your master chain again, so remove the routing (set to ‘no bus’) and use the control room/listen bus (press L on both, mixdown and master channel, listen dim set to - infinity).
The smaller Cubase versions don’t have it, for the sake of testing set up a project without any plugins in the master channel.