The amount of xSUSHI in the SushiBar will not change. But the amount of Sushi it represents will.
As fees are collected they are converted to $SUSHI and distributed proportionally across holders in the xSUSHI pool. So when you withdraw your xSUSHI back into $SUSHI it will be worth more $SUSHI than when you put in.
These rewards are slower to update (because they cost a lot of gas) and so you should expect them to update, on average, once per day.
Currently there is no way in the UI to see the level of rewards until you withdraw. The team is working to fix this.