I thought based on the description that Loki was taller Giant than normal Giants..
Was he said to be in the manga? Then he will be.
There are other factors here.
- Oda isn't consistent with sizes.
- Luffy is closer to the "camera".
- Loki is drawn in perspective.
He didn't seem that much bigger than a normal giant when he was tied up, though, but we will see how he compares to Hajrudin next chapter.
My personal opinion is that normal-size giant is good enough. Just let him be a "superhuman" normal-size giant.