That’s not saying much
Definition 1 of 1
Pro Tip 1/3Understated Impact
Implies the achievement or fact doesn't make a significant difference or change opinions. 
She did better on this test, but that's not saying much since she usually scores below average.
Pro Tip 2/3Comparative Context
Used when comparing, suggesting there's a low standard or poor performance generally. 
This is the least busy day at the store, but that's not saying much given our slow month.
Pro Tip 3/3Mild Disparagement
Can be a polite way to criticize, indicating something is slightly better but still inadequate. 
It's cleaner than before, but that's not saying much considering the mess it was in.