UHPLC columns are contingently more efficient. You will get the same separation when scaling down linear from HPLC to UHPLC:
250 x 4.6 mm ID (5 µm particle) → 150 x 4 mm ID (3 µm particle) → 100 x 2 mm ID (2 µm particle)
For more effiency relating to theoretical plate numbers the separation distance has to be longer as used for linear scale down.
Furthermore if the extracolumn contributions to band broadening of a particular HPLC hardware system are high then the full performance of this column will not be realized.