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QR code has very limited data capacity. (According to Wikipedia) one single QR code can only carry <3KB of binary data.
It's not rare for a PSBT to hit this 3KB limit, especially for ones which spend non-SegWit UTXOs.
SegWit features signing of input values - on the other hand, signing non-SegWit inputs on offline/hardware wallet still requires full data of previous transactions to validate input values. Otherwise, it could be a security issue: a malicious party/malware could tamper the inputs stealthily to trick the user into paying an unexpected high amount.
A solution is not using QR codes for an inappropriate purpose. Find something else. – Anonymous – 2019-11-11T03:34:47.007
Use multiple QR codes – Andrew Chow – 2019-11-11T03:41:18.860
@AndrewChow Multiple QR codes seems to require something like sequence number/checksum etc, maybe fountain code would be better. – Chris Chen – 2019-11-11T06:50:46.700
@Anonymous I think at least QR code is better than USB, because I'm concerned about ADB. ADB has been "abused" by various companies, including both Android handset vendors and software vendors for quite a long time, they had turned ADB from a debugging interface into something for everyday use (as something similar to iTunes), thus countless users had already carelessly left ADB enabled. – Chris Chen – 2019-11-11T07:07:08.323