I expect it's the flowers that are meant to be giant, not necessarily the tubers. That does look a bit puny, but it should grow and build up for future years. New tubers come in different sizes (for example https://www.jparkers.co.uk/how-supplied/summer-bulbs-corms-tubers/begonia-tubers/ )- bigger ones will be more mature than smaller ones and therefore cost more. If you paid for tubers of a particular size and what you received are all smaller, it's worth an enquiry (with your pictures) in case they made a mistake in picking the order.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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