Tangents are non-weighted by default in Maya, which means they are locked and you can only change their angle, not their length. If you want to adjust tangent length by dragging them, you have to change tangent type to Weighted.
Non-weighted tangent: you can change angle but not length
Weighted tangent: you can change angle and length
Non-weighted tangent (triangle-shaped) handles adjust only the tangent angle, the tangent length is locked and non-adjustable, while weighted (square-shaped) tangent handles let you adjust both the length and angle. Changing the length of the tangent adjusts the ease in and out of the position.
To change the tangent weight