This size is not uncommon with indexes, especially for large text fields. While it might be indicative of a poor indexing strategy, text is notably more complex to index than numbers, especially if enabling substring searches not anchored to the beginning of the text field.
While a large index might seem slow by its nature, it can (or should be) navigated quite quickly due to its highly structured nature, and thus yield pointers to the correct data faster than an exhaustive search of the data itself.
Assuming they will continue to fit on disk, the real tests of your indexes are satisfactory search, update and addition times, especially as the amount of data scales. If these aspects continue to be adequate, a large index is tolerable.