The focal ranges are more intended for APS-C, but do check out the Sigma 19/2.8 and 30/2.8 lenses. They are not quite a pancake lens, but they are pretty close.
Are you OK with manual glass? If so, check out the Samyang 135mm f/2. If that's still too big, the Pentax 135mm f/2.5 or Nikkor 105/2.5 are both quite compact (roughly the size of a can of soda) and will provide you with a little more reach. None of those is a native M4/3 lens, so you will need to buy them in a Nikon or Pentax mount and use an adapter.
Are you OK with manual glass? If so, check out the Samyang 135mm f/2. If that's still too big, the Pentax 135mm f/2.5 or Nikkor 105/2.5 are both quite compact (roughly the size of a can of soda) and will provide you with a little more reach. None of those is a native M4/3 lens, so you will need to buy them in a Nikon or Pentax mount and use an adapter.