The maximum resolution limit on Mapillary is 108 MP (108,000,000 pixels to be exact).
I do not know about Yandex but Mapillary renders in the web app viewer the full image in a single texture only. On certain GPUs, especially often on mobile GPUs, the maximum texture size limit is below the image size in any dimension or Mapillay’s maximum limit. In this case, the image is scaled down to the maximum supported texture size limit for display. For 360° images, particularly in the width dimension the GPU texture size limit can often be lower than the original image’s width or Mapillary’s limit of 14,696 px. Hence, the viewer will never display images more detailed than the renderer (the GPU) you are using is capable of. This is a known limitation of the Mapillary web app viewer.
Certain browsers let you use a software renderer, which supports either unlimited texture sizes or the minimum spec size of 16,364 px. This way, you can view Mapillary images in full detail even on limited texture size GPUs.