: Mass doubling for a healthy man from marathon training (summer) to weight training (winter)? Normal? I have trained a variety of sports such as soccer, swimming, skiing -- and later in the university got interested in weight-lifting.

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I'm going to restructure your question a little bit to help you understand what's going on:

Is it normal for mass to increase substantially going from marathon training to a hypertrophy based resistance training?

The short answer is: yes. The longer answer has to do with the specific adaptations your body is going through in response to the training. Ignoring all cardiovascular changes (and there are many in this category), the primary difference from a body composition standpoint is:

Endurance based training develops type I muscle fibers (AKA slow twitch).
Hypertrophy based resistance training develops type II muscle fibers, primarily type II-B (AKA fast twitch).
It is not uncommon to add fat mass while intentionally bulking.

There are several differences between slow and fast twich muscle fibers. Much of those differences have to do with the energy systems they are designed for. Slow twitch muscle is optimized to work from the aerobic metabolic pathways. They fire more slowly and use oxygen and ATP to fuel work. Fast twitch muscle is optimized to work from anaerobic metabolic pathways. They fire quickly to produce force and use glycogen and ATP (with lactic acid byproducts), or the phosphogen system (using creatine phosphate to replenish energy).

Additionally there are two primary types of hypertrophy:

Sarcoplasmic hypertrophy--energy support systems (glycogen reserves)
Myofibrilar hypertrophy--muscle contractile protein pairs

As you perform hypertrophy based resistance training (typically emphasizing sarcoplasmic hypertrophy), your body makes adaptations to increase the potential energy your muscles can store, and then fills those energy reserves. All of this increases your mass. Resistance training is also more anabolic than catabolic. Essentially it's telling your body to get bigger and stronger for short bursts.

As you perform endurance based training, your body makes opposite adaptations. Since endurance uses the oxygen based energy systems, it no longer needs the same glycogen reserves in the sarcoplasm. Additionally, since the body wants to be efficient for the job at hand it needs to reduce mass. Your type II muscle loses its sarcoplasmic hypertrophy, and starts getting converted to type I muscle which takes up less mass. Endurance training is more catabolic than anabolic. Essentially it's telling your body to get lighter move longer distances.

Assuming your body fat stays the same year round, you will still have more mass after extended hypertrophy based resistance training than extended endurance based training.