Arm pit is quite a common place where lymph nodes and cysts show up quite commonly as it is an area that sweats a lot. Anything that affects the process of perspiration or an infection in the nearby area can produce a swelling. Even usage of antiperspirants can cause lumps. Normal swellings, however, will be skin coloured. Since you have mentioned the colour as white and it is growing in size, may be you should consult a doctor and possibly get it removed.
This may be a sebaceous cyst. However this would need to be confirmed by a doctor.