Context: LowLevelHelpers.sol#L110
This is not compliant with the ABI standard, as the ABI standard allows for returning more data than needed. The proper one should if iszero(lt(returndatasize(), OneWord)).
This is currently used in 
The ABI standard allows for extra data everywhere. That is, given a correct 32-byte value, having more data at the end is valid. Therefore, this function returns false for complaint contracts.
However, most contracts wouldnt return more than 32-bytes when only 32-bytes are needed. A notable exception is some versions of proxy contracts in Vyper. This proxy always returned 128 bytes, with any extra data padded with zeros (source). This has caused issues with SolMates SafeTransferLib, leading to DOS issues on chain (source).
This is at least a low severity issue, and may even be considered Medium if there are known EIP1271 wallets in existence with the aforementioned issue. Note: for ERC20 tokens, there are known tokens with the problem (certain Curve tokens). Considering that this is a trivial fix, its better to be on the side of caution and make the above change.
For a simple proof of concept, consider a minimal proxy contract that does return(0, 0x1000) (instead of the usual return(0, returndatasize())) where the implementation is a EIP1271 wallet contract. The low level helper would revert when verifying the _assertValidEIP1271Signature, whereas a high level solidity implementation would succeed.
Change the strict equality check to a >= check.
