In the campaign games I have played in (using VBAM for the economy), we have always used the SU as a calculation for mass. So a 2HS ship with 247 SU is more than twice the size of a 1 HS ship with 100 SU.
Using the HS^(2/3)*constant, with the constant set at 100 (to match SU at HS1) gives:
HS SU HS^(2/3)*constant
1 100 100
2 247 158.7401052
3 418 208.0083823
4 607 251.98421
5 811 292.4017738
6 1,028 330.1927249
7 1,255 365.930571
8 1,493 400
9 1,740 432.6748711
10 1,996 464.1588834
11 2,259 494.6087443
12 2,529 524.1482788
13 2,807 552.8774814
14 3,091 580.8785734
15 3,381 608.2201996
16 3,676 634.9604208
17 3,978 661.1489018
18 4,285 686.8285455
19 4,596 712.0367359
20 4,913 736.8062997
21 5,235 761.1662611
22 5,561 785.1424411
23 5,892 808.7579399
24 6,227 832.0335292
25 6,567 854.9879733
26 6,910 877.6382955
27 7,258 900
28 7,609 922.0872584
29 7,964 943.9130677
30 8,323 965.4893846
Using SU as an indicator of size (mass) makes the big ships a lot more massive.
/end ridicule