If massive animals live longer, why do humans with gigantism die younger?
In short many things determine life expectancy and the interplay between different factors will determine an individuals life expectancy and a species' average life expectancy.
How life expectancy links to size:

Among mammals there is an inverse relation between heart rate and life expectancy (mammals tend to average 7.3+/15.6 x10^8 beats per lifetime which appears to be a symptom of the fact that heart rate is a marker of metabolic rate and so there is a fixed 'metabolic lifetime' for animals (source). This is somewhat substantiate by metabolism variants in C. elegans and the link between metabolism and free radicals and DNA damage (source).
Larger animals are thought to have a slower metabolic rate by weight (not overall) due to scaling laws (see Kleiber's law) and how surface area and volume relate and change temperature regulation metabollic requirements.

Other factors that change life expectancy:

In breeding: a reduction in genetic variation caused by inbreeding (e.g. many dog species though the extent to which they are inbred will depend on the species) can lead to lots of underlying health issues that may reduce the lifespan independent of metabolic rate
Stress: Higher stress will reduce life expectancy
Nutritional abundance: reducing caloric intake may reduce metabolic rate and extend lifespan, though starvation will decrease lifespan

Gigantism:

Whilst the life expectancy for a species may be higher in general people with gigantism may live shorter lives than possible because of non-metabolic related reasons. For example many forms of gigantism are linked to hormones which promote cell division and growth which are also linked to forms of cancer.