In general, power-to-weight ratio (in terms of the power and weight of just the engine itself) is not a very important property in a tank engine (compactness is much more important). This is because the performance of the tank is determined by the ratio of the engine's horsepower to the overall weight of the vehicle, of which the engine makes up a relatively small fraction (the Maybach engines in the Panthers and Tigers weighed around 2600 pounds, compared to the 50 ton and 60 ton overall weights of those tanks - 2.6% and 2.2%).

So the engines had (relatively) poor power/weight ratios because there was no reason to put a more expensive, lighter-weight engine into a tank. Brief tangent: an interesting thing that the British did in WWII was to power many of their tanks with the Meteor engine, which was basically just a Merlin without all of the weight-saving optimizations (usually using plain steel in place of aluminum, no supercharger etc.). For more than a year, in fact, they were able to build their Meteors using engine blocks recycled from war-weary Merlins, which is a bit of industrial efficiency I'm not aware of any other combatant making use of.

A bigger problem with the German tank engines was that they were gasoline-powered, which made the vehicle a severe more of a fire hazard (as was true of American and British tanks as well). Russian tanks generally ran on diesels and so were able to carry less inflammable fuel (an obvious benefit when you're being shot at regularly). German commanders were well aware of these advantages and wanted their factories to copy the Russian engines (in fact they wanted copies of the entire T-34 when it first appeared) but this was impossible thanks to their general aluminum shortage.

Edit: I interpreted this question as referring to the power-to-weight ratio of the engine itself. If OP actually meant "why did a 60-ton tank have just a 600 hp engine?", this was because even though the weight of a tank engine doesn't matter very much, the space taken up by the engine is critically important. A Tiger would certainly have benefited from a more powerful engine, but a more powerful engine would have been larger and required larger fuel tanks, which would have meant a larger vehicle, which would have meant more armor, making the vehicle heavier and easier to hit. The Tiger in particular was already severely pushing the upper size limit in terms of the ability of their transportation infrastructure (and the ground, literally) to support the vehicles. So the designers selected an engine that gave just enough power to move the vehicle at an acceptable tactical speed, and no more.

Regarding gasoline vs. diesel: gasoline is more volatile than diesel fuel and has a lower flash point, so if vapors have accumulated in a partially-filled tank, the gasoline tank will be more likely to ignite as a result of a projectile/fragment penetration, although this risk differential can be mitigated to some extent by proper venting of the fuel tanks.