There are a few things that can affect the internode length.
1. lack of nutrients. It may be related to pH that makes nutrients unavailable or because your fertilizer is organic, just not having enough available nutrients when the plants need it. Or it can be too much of one nutrient inhibiting uptake of another.
2. Internode length varies with season early season = longer internodes. late season = short internodes
3. if there are a lot of branches and stems then the plants energy will be diverted or divided leading to shorter internodes and short plants if nutrient levels are marginal.
4. plant growth regulators and herbicides can change internode length.
5. Temperature, light, humidity, CO2 are all considerations.
"Final stem length is determined both by number of internodes and internode lengths (Pearson et al., 1995). In species with a determinate growth pattern, such as chrysanthemum, new internodes are formed up to flower initiation. After this stage, the increase in stem length depends on internode elongation only. Thus, the stem elongation process is strongly correlated with both internode appearance rate (IAR, equal to leaf unfolding rate) and internode elongation rate.
Several growing conditions are known to affect chrysanthemum stem elongation, such as temperature, light intensity, light quality, photoperiod, relative humidity, CO2 concentration and plant density (Carvalho and Heuvelink, 2001)."