Think about what's going on inside the crankcase.

When the piston moves up the cylinder towards the head the volume inside the crankcase increases causing a low pressure inside the crankcase.

When the piston moves down the cylinder away from the head the volume inside the crankcase decreases compressing the air resulting in a high pressure area inside the crankcase.

The vent tube compensates for this change in pressure. When the pressure inside the crankcase increases it will be vented out through the tube. When the pressure decreases the vent allows outside air in balancing pressure through the vent tube.

During this "breathing" process fuel residue inside the crank will be pushed through the tube. If the tube is too long the fuel will create a plug inside the vent tube that will adversely effect this breathing.

Another affect of a "too long" tube is that this pressure buildup will push the fuel residue out the front bearing as it is not sealed. Result: one really, really slimy engine & engine compartment.