Learn Finnish: Kasvu oli valtava ennusteisiin nähden. - The growth was huge relative to the forecasts.

The postposition nähden (meaning 'relative to' or 'in view of') always governs the illative case. You can remember this by thinking of its root, the verb nähdä (to see): it is historically the instructive form of the verb, so it carries the feeling of 'looking into' the forecasts.

How is the plural illative ennusteisiin formed from the base word ennuste?

ennuste (forecast) is a word ending in -e, which means its vowel doubles in the stem: ennustee-. To make it plural illative, you add the plural -i-, creating the diphthong -ei-. For words with this stem type, the standard plural illative ending is -siin, giving us ennusteisiin.

Could I use verrattuna instead of nähden in this sentence?

Yes, you could say ennusteisiin verrattuna ('compared to the forecasts') and the meaning would be practically identical. Both nähden and verrattuna take the illative case. nähden often translates better to 'in relation to' or 'in view of', while verrattuna is strictly 'compared to', but they are very often interchangeable.

Why is the adjective valtava in the nominative case here rather than the partitive?