This does appear to be an overcrowded area. The residential district building lot size is 7000 square feet, but many houses there have been built on lots far less than that size. This includes even some recently constructed homes.
With the snow storms this past winter, snow banks became a real problem in these crowded neighborhoods. Parking has also become an issue in other locations in the city where the properties have been built on much smaller lots than what the zoning codes currently require for new building lots.
More information about a street can be found by clicking on the real estate appraisal link on the main Waltham city website, and putting in the street name without a specific address. This can give an indication as to what recent properties have been built, the lot size, and house and land valuations. Also, additional information, such as the zoning district, can be found on the Building link on the main page, and then clicking zoning map. Finally, if there seems to be a concern about pending construction causing flight in a neighborhood, you can check on the building department link for any pending building permits for new or expanded construction in the neighborhood.