Analyzing the code you provided (I have added packages that are needed):
require(dplyr)
require(flextable)
regulartable(mydataframe) %>% theme_zebra() %>% autofit()
I can see that you are using autofit() function, which sets the height and width of the columns automatically to fit best to data in the dataframe, and as I understand - you do it separately for each table. To solve this formatting issue, you don't need to concatenate those dataframes, but rather you could set the same width of columns for each table, which would look like this:
regulartable(mydataframe) %>% theme_zebra() %>% width(width = 1)
Depending on what kind of report you are creating, you should consider what type of data will be displayed in your tables and adjust the width parameter in the width function. It is worth mentioning that the width parameter takes a number representing the number of inches that your column should be.
I strongly recommend you reading documentation of flextable package to get further information on formatting those tables of yours.