If you go to Settings > General > Storage & iCloud Usage > Manage Storage > OALD on your device, it suggests that OALD is a very small app. This is because the device’s storage settings only take into account the size of the OALD container app, not the in-app purchases within it.
The OALD container app alone is approximately 70MB, but the in-app purchases of OALD8 and OALD9 are around 500MB and 750MB respectively. OALD8 and 9 are very large dictionaries with a lot of audio and picture content which is why they require this much space.
The Documents & Data figure relates to your Favourites and History. If you have only just bought the app then this number will still be low.
The spoken sentences available to download in Settings > Sound are also not taken into account in the device’s storage settings. The sizes of these soundfiles are as follows: BrE sentences = 524MB, AmE = 520MB.