This video on youtube shows a giantess taking a bath when all of a sudden a tiny woman appears. The giantess is unaware that the tiny woman is there so the tiny woman takes this opportunity to study the giantess in a tiny swimming pool. With the information she needs the tiny woman builds a giantess robot that looks like a giantess and once she is happy with it she joins the giantess robot on a salvage mission to recover something from the bottom of the sea. It take a while of searching but eventually she finds what she was looking for. When she resurfaces she encounters a giant robot but because she is accompanied she easily defeats the robot and sends it flying in the distance.
You can watch this video here Kirameki Giantess scenes