Does the Enlarge Spell Make Sense?

Some players criticize the enlarge spell (PH p. 200) for not giving the full benefits for an increase in size category such as increased reach and abilities (MM p. 12) -- or worse, simply assume (incorrectly) that they get those benefits. In truth, all the recipient of an enlarge spell gets is +1 Strength per +20% of growth, and it makes sense that this is the rule.

First of all, it's important to note that increase in size category assumes a full doubling of size (MM p. 5). No one should get any bonuses at all just because they've defined their character to be 3'-10" and then grow an extra 4"; even though this ostensibly puts them out of the range of the "Small" category and into "Medium", their ability scores are all based on a nearly-Medium build, and so they require a full doubling in height to get the modifiers for an increase in size category.

For enlarge, what the designers apparently did is to take all of the benefits of size increases, which vary depending on initial size category (as shown on MM p. 12), average the effects across all size categories (so as to have a single, simple rule), and then pro-rate the increases for the various levels of the enlarge spell. Doing this pro-rating, from 10% to 50%, and rounding down per normal D&D rules, comes out with almost exactly the rule given for modifiers from the enlarge spell (as shown in the table below). Abbreviating the rule by ignoring the minimal extra adjustments at the 50% category (for Con and AC/Attack) is a sensible streamlining. Furthermore, this analysis stays basically the same even if the range of starting size categories is restricted to Tiny-to-Large, or even just Small-to-Medium.