Q.Improved Meyer Lemon tree
Why does my Improved Meyer Lemon tree have two giant pieces of fruit on it? They are growing on the same branch. They are almost the size of basketballs. The rest of the fruit is of normal size. This is a healthy tree and has always produced well.
Certified GKH Gardening Expert
If the branch sprouts off from the base of the tree or below the graft ring, you could be seeing the rootstock trying to regrow. Typically these kinds of branches should be removed from the tree because the rootstock fruit is inferior to the scion (which would be the Meyer's lemon for you).
This article may help:
https://www.gardeningknowhow.com/ornamental/trees/tgen/can-grafted-trees-revert-to-their-root-stock.htm