Yes, you can. You can hold large XRP balances safely in XUMM starting version 0.5.1
We are confident that security, encryption, storing, sending & receiving XRP in XUMM has been thoroughly tested. We're improving the user interface here and there, adding features & translations & sending, receiving & trading IOU's (non-XRP currencies) are work in progress.
This means we trust XUMM can handle larger XRP balances from version 0.5.1 and up.
However, and we can't stress this enough: ⚠️ KEEP YOUR KEYS SAFE. WRITE THEM DOWN & STORE THEM AT MULTIPLE SECURE PLACES ⚠️