When setting the ShowUpDown to "false" on the RadzenNumeric, I would expect that the padding of the component would also adjust and give more room for the numbers.
What I see however is that even set to false, the padding is still there reserved for the arrows.
Here we can see the two settings having the same dimensions on the purple padding
Since you are applying some custom CSS you might find this useful:
I'll give it a try, but in this case there is no custom CSS to the element added
This doesn't happen as it would require additional code to run (toggle a class that removes the padding). We think it is a bit of over-engineering. You can use CSS to remove that padding if you don't want it.
Fair enough 
from a user side, that's how I expected it to work, but can understand the overenginnering concern as well 
If the two pixel padding is bothering you for some reason use CSS to remove it