Here I tend to think differently.
There are at least 2 points against small bundles:
1. Portability; Means if you create a several tens of gigabytes sized with hundreds snapshots repo, it will span into tens of thousands of bundles, making it very hard to move to different media. It becomes a show stopper if you use slow or NAND based (but non-SSD) external storage.
I do agree large bundles makes restoration REALY slow, but I guess we can review restoration mechanism.
I still didn't review in details restoration process but I guess utilizing bloom filter might help to perform intelligent and linear data extraction.
2. Repository size; My experiments show that large bundles (32-128MB) improve compression by ~5-15%, which might be preferable despite longer backup time.