These days tanks have incredibly advanced armour and defensive options. Yet there is one main tactic used. "Hit first, so they cannot fire at you." The reason for this is very simple. Firepower is so big these days, that most hits are a guaranteed bad time.
Big vehicles can leverage this. Their armour might be able to be thicker, as well as their defensive arsenal of misdirection. However, their offensive capabilities and detection systems are where it's at. They can have bigger, further ranging weapons than their smaller counterparts. Together with their superior detection, or via the detection capabilities of another unit in the field, they can pinpoint and calculate the trajectory of a long distance shot. Then they can destroy the enemy before it gets in range. They can have a huge supplementary (or main) arsenal of rockets, autocanons and huge rapid fire guns. This allows further staying power against any air or ground assault, as well as some defensive capabilities against flying armaments.
They might not go in the front line, but their support capabilities will be impressive.
The big problems
Being big comes with a host of problems though. Like the mause in your example, it is very bad for the ground. Roads get damaged, it can easily sink in softer ground, the wheels/tracks/other that drive it forwards need to be of special matetials and can still wear quickly. This immediately presents a vulnerability. It's movement is restricted by bridges their size and weight capabilities, the width of roads, whether or not you want to damage/destroy roads by just driving on them, soft ground, forests and more.
It also presents an easy target. Despite a huge arsenal of anti detection measures and a whole lot of camouflage, it's still big. It can be detected relatively easily. If it goes through a forest the trees are likely not much if a problem, but you can also follow the behemoth easily from the destruction of just driving. That will attract an airstrike or heavy weapon fire rather quickly.
It also represents a large cost. The components, weapons and even just maintenance, not to mention fuel cost, will skyrocket. This is even without it getting fired upon.
The fact is that a small devision of specialised vehicles likely offers more firepower, mobility and stealth than one behemoth. As well as a distributed intelligence gathering, the one big tank is unlikely to add it's worth when added to a battalion compared to several smaller specialised ones.
Strong, capable, far reaching, difficult to take down. Easy target, high costs on all flanks, limited mobility that can destroy even when you don't want that, easily replaced with a host of cheaper, specialised units.