Your plants may have been incorrectly labeled, says UC master gardener Bill Pierce. Dietes bicolor and Dietes vegeta often are confused. D. bicolor grows to 2 feet and D. vegeta grows to 4 feet. These plants grow from rhizomes, and removing rhizomes can control the width of the clump. A rhizome is a bulblike structure, an underground stem, from which the roots and the leaves grow.
Dietes are tough plants and can withstand drastic pruning.
If you cut the tops back, new 4-foot leaves will grow, and you also might be cutting out the immature flower stems. Let us hear from you.