Assuming this is a point layer containing features which are ordered accordingly, you could create a field containing decreasing values. Then use this field for the
Size setting in the layer's Style Properties.
Depending on how your layer is structured (i.e. if it contains several groups of points), you may need to select your points carefully and then either manually enter a size in the attribute field or use an expression.
Assuming this is a line layer, you could use the Arrow symbol layer type and set it so that the
Head length and
Head thickness have a value of 0. And then set the
Arrow width at start and
Arrow width settings to play with the size:
This could create a line of varying size: