I have tried to add the &z=15 and %zoom=15 to try to edit the default zoom in the embed code but it seems to not change anything. Also, I have changed up the width and height of the embed code but that didn't change the zoom just the overall dimensions. I am trying to embed a flowchart into my company's intranet SharePoint but the zoom is always set to 19% or 24% by default which makes it impossible to see anything in the diagram by default. I want it to start zoomed in at like 100% so the website/diagram is easier to use. I have also tried to print it in a pdf, converting it to pdf, and converting it to other formats for the file viewer to give different views but it came to no avail. The closest thing was when I changed it to pdf and it gave me a better view, but I lost the interactive embed that a normal Visio embed would give. Thank you for your help!
Puzzled with a similar problem, I've built a special Web Part for that (it provides additional options for embedding that are not part of the stock web part, such as zoom).
The source code: https://github.com/nbelyh/VisioOnlineSpfxWebPart
Here web part in MS Store: https://appsource.microsoft.com/en-us/product/office/wa200002491