I think the opposite of "diminutive" is "augmentative" (увеличительный). For some reason, the form does seem to be rarer in Russian than the diminutive.There are a few augmentatives formed with suffixes, including -ище, -ишко, and -ин-, for example: дом (the house) домище (great house) домина (huge house). An impression of excessive qualities is sometimes made using the suffix -га, for example: ветер (the wind), ветрюга (strong wind).
There are neuter nouns ending in -ище and declined substituting the usual -o with -e. (чудовище, чудовища, чудовищем, и т.д.)