So i suppose it depends on what ever training your doing as to how your physique takes shape to benefit the most.
Right - S.A.I.D (specific adaptation to imposed demands)
The likelihood that cycling results in any increase in upper body /upper limb size I find a bit questionable (not because I am disbelieving people's experience but rather I think there is a different or better explanation.)
To increase muscle mass /size / strength you need to trigger myofibrillar and / or sacroplasmic hypertrophy (although you can see an improvement in strength levels without necessarily increasing size by improving the ability of your central nervous system to handle load - neuromuscular adaptation - but at some point it needs an increase.)
To do so requires a sufficient load and to keep doing so requires the load to be progressively increased which is something that generally does not present itself in cycling in relation to the upper body even with repetitive straining and pulling on the handlebars (although there is a possibility of an increase in leg size, particularly quads and glutes to a limited degree IIRC, for new cyclists as the level of resistance they find themselves working against initially in say hill climbing is possibly high enough to cause micro tears in muscle fibres and therefore hypertrophy.)
