Why are endurance bikes so bulky or wide nowadays?

I disagree that Endurance bikes look "huge". Taking your example of Canyon, the current model of the endurance has very slender tubes by modern standards.

However there are a number of reasons why an endurance bike may be built slightly heavier and wider than an equivalent race bike.

Typically riders of endurance bikes will be looking for features that facilitate longer rides, in more remote locations, over a wider range of surfaces and at lower speeds. These requirements lead to a number of features that may be found on endurance bikes, such as lower gearing, wider tyre clearance, extra bottle mounts, mudguard mounts and extra shock absorbing features.

Second moment of area increases with distance to the power of 4.

This means a tube with a wide diameter and thin walls is much stronger against bending or torsion than a narrow tube with thick walls and the same mass.

It’s especially noticeable in the down tube and bottom bracket area. But this shouldn’t be something you only see on endurance bikes.

What’s different for some endurance bikes is that they have more tire clearance (leading to wider forks and seat/chainstays). Some also have built-in tool compartments or other features which could add width.

Race bikes might try to reduce frontal areal to improve aerodynamics. Hence why time trial bikes often have narrow but tall tubes. Or flat but deep handlebars.

Edit: Regarding wall thinness: Of course there are limits for wall thinness. Tubes mustn’t buckle like a beer can, you should be able to sit on the top tube and the tubes in general have to be able to survive some impacts and abuse. Another limit is manufacturing since thin tubes are harder to weld (in case of metals) or layup (in case of carbon). One of the advantages of lower density materials like aluminium and titanium is that you can have wide tubes while keeping relatively thick walls.