The encoding of a varint is answered in this comment. It's not a straight hex string to decimal conversion.
From the specification:
Varints are a method of serializing integers using one or more bytes. Smaller numbers take a smaller number of bytes.
Each byte in a varint, except the last byte, has the most significant bit (msb) set – this indicates that there are further bytes to come. The lower 7 bits of each byte are used to store the two's complement representation of the number in groups of 7 bits, least significant group first.
Therefore, you first drop the MSB of each 8 bit group then reverse the order of the remaining 7 bit groups (because varints store numbers with the least significant group first).
So in the case of
FFFFFFFFFFFF03 = 11111111111111111111111111111111111111111111111100000011