Originally posted by FrothByte
1. Is easily answerable. Because Thanos defeated and overpowered characters like Hulk and Thor, so we can easily say he's stronger than them and can replicate their strength feats and more. WW never overpowered or defeated Superman, so it's very difficult to quantify just how strong she is based on that headbutt. We can use her tank lifting feat to quantify her strength, but that still makes her weaker than Hulk.
But you do know about fiction inconsistency right?
1. Weaker characters can and have struck harder than stronger characters.
2. Characters have highs and lows. For example, a character can have a million ton feat (supergirl) and perform under 200 tons on average. So someone overpowering supergirl in one scene doesn't mean they too can lift a million tons. It's just means that they are stronger than her average and everything else is fiction inconsistency.
Therefore Stronger characters don't get weaker character's top feats. Thanos is stronger than average Hulk. Thanos can't leap like the Hulk because his legs aren't as strong. Striking and lifting, although positively correlated, are different. As others pointed out, an expert and experienced fighter can strike harder than someone who is both a lot stronger and has little to no fighting experience (a bodybuilder for example).
In other words, Thanos being stronger than the Hulk does not mean he can headbutt harder than WW. Visually WW headbutts harder.
Lastly you are giving Thanos strength due to the people he fought. Well WW headbutt Superman and rocked his head back. Given Superman's feats then that is a strength feat well beyond lifting a tank. So you should judge WW's strength off that and not other feats.