The Collaborative International Dictionary of English v.0.48:

Proportionally \Pro*por"tion*al*ly\, adv.
   In proportion; in due degree; adapted relatively; as, all
   parts of the building are proportionally large. --Sir I.
   Newton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

proportionally
    adv 1: to a proportionate degree; "your salary will rise
           proportionately to your workload" [syn:
           proportionately, proportionally] [ant:
           disproportionately]