Translation:It is not heavy, you know.
I'm not a native Japanese speaker so I may be wrong, but I'm pretty sure you could say this to someone who does not in fact know that it isn't heavy. In that case the English translation seems to imply that the person ought to know, introducing all kinds of tone that wasn't specified in the original text.
There are lots of ways in which the phrase "you know" is used in English. Sometimes it's used for emphasis, but language can be so complicated, you know! Other times it's, you know, like, filler words. Or I think it could be used as a request for acknowledgement, you know?
I don't think "you know" is a very good default translation for よ because its function is so context dependent. To me, that particular English sentence reads as either caustic or vague.