Every spring when the snow melts and I hit the dry trails I have a feeling of intense strength in my legs, butt, and core, particulary through my quads and glutes. My pants fit a bit tighter in the legs/butt and my weight is few pounds heavier. I attributre this to the differences in my training and racing through the winter.

First of all during the winter I train and race on snowshoes. The available grip provided by the snowshoe bindings allows me to climb/push up steeper pitches ie. ski slopes. This increases the strength and mass of my quads. This same grip along with the soft foot landing in snow allows me to descend steep, deep powder slopes at top speed. I believe that this is the most significant contributor to my leg/butt stength of any training I do. It takes an incredible amount of strength to control yourself at top speed while descending. Snowshoeing through deep, singletrack powder requires an immense amount of core and leg strength in order to maintain balance and appropriate leg lift. Also the average length of snowshoe races I complete is 60 minutes (10K). This is a much more intense effort compared to my longer races (3-5 hours) of the summer.
Secondly, during the winter (especially this winter) I nordic ski as a form of cross training. The main benefit for me from nordic skiing is the core strength. Like snowshoeing I use my core for balance/control and for extra power during poling. As well it increases leg and butt strength comparable to biking during the summer. One thing I noticed this winter is I have to be conscious of using both legs equally while skate skiing. My right leg tends to develop a bit more strength since I tend to use it more. This leads to compensation during running in the spring. Finally, I develop a bit more spring in my legs, perhaps more fast firing muscles from skate skiing.
So the moral of the story is I believe that a strong winter season is where I gain the majority of my strength that prepares me for long, sustained climbs and descents during trail running in the summer. This combined with biking as cross training during the warmer months keeps the quads and butt strong. I strengthen my core in the gym with medicine ball and kettle ball workouts during the summer and prior to the winter season.