解:$(1)$黄瓜:$300 ×30\% = 90 (\mathrm {m^2})$
油菜:$300 ×20\% = 60 (\mathrm {m^2})$
西红柿:$300 ×35\% = 105(\mathrm {m^2})$
芹菜:$300 ×15\% = 45(\mathrm {m^2})$
$(2)$黄瓜总产量:$90 ×7 = 630 (\text{kg})$
西红柿总产量:$105 ×8 = 840(\text{kg})$
$(840 - 630)÷840×100\% = 25\%$
答:黄瓜的总产量比西红柿的总产量少$25\%。$