Considering that prefix "pseudo-" means "false/fake", then a pseudo-giantess becomes a fake giantess. That is, it is about a woman who, although she has never grown in size (relative to the universe around her), appears to be a giantess.
There are several cases in which pseudo-giantess can be presented, see:
When a woman interacts with beings smaller than herself:
Pokémon - EP0022: Sabrina shrinks the protagonists. She has never grown in size (so she is not a giantess), only interacted with shrunken beings (Ash, Misty, Brock and Sabrina's father). That's why she's a pseudo-giantess.
A sand castle pretends to be a structure created by humans in miniature, hence, the woman who interacts with this castle seems to be a giantess.
In this case, we can deduce that the train is a toy because the stage is a common room, then that woman is a pseudo-giantess. But, if the stage changes from being a "room" to being a "city", then she will be considerated a giantess.
The interaction of an ordinary woman with a scale model produces a pseudo-giantess.
Among the best-known models, there is the anime figures. here we have a cosplay of a pseudo-giantess Madoka.
When you stretch the image of a woman because of the presence of a large screen or a huge advertising poster:
On Shinzo - EP26, appears the image of a woman in the bottom of the scene. This woman never really grew up, she simply interacted with the protagonists through a "giant screen" (at the bottom of the stage), making her lookgiantess. Also, this a case of pseudo-giantess.