Sketchup as you say includes referenced models in the main file, not just a link to one or more external files.
To minimise file size, you may have to reduced the complexity of plant sub-models. Most of the ones on the 3D warehouse are colossally over detailed for inclusion as entourage, often with millions of edges and file sizes in multi-megabytes, and detail down to individual leaves.
You get more manageable ones with 2 1/2D versions - images with transparent boundaries, perhaps in two or three intersecting planes, showing a view from different directions.
Search for XXXX low poly where XXXX is the kind of plant you want